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629 lines
19 KiB
629 lines
19 KiB
// SPDX-License-Identifier: GPL-2.0-or-later |
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/* |
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* Copyright (C) 2010-2017 Mathieu Desnoyers <[email protected]> |
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* |
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* membarrier system call |
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*/ |
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#include "sched.h" |
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|
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/* |
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* For documentation purposes, here are some membarrier ordering |
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* scenarios to keep in mind: |
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* |
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* A) Userspace thread execution after IPI vs membarrier's memory |
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* barrier before sending the IPI |
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* |
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* Userspace variables: |
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* |
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* int x = 0, y = 0; |
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* |
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* The memory barrier at the start of membarrier() on CPU0 is necessary in |
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* order to enforce the guarantee that any writes occurring on CPU0 before |
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* the membarrier() is executed will be visible to any code executing on |
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* CPU1 after the IPI-induced memory barrier: |
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* |
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* CPU0 CPU1 |
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* |
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* x = 1 |
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* membarrier(): |
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* a: smp_mb() |
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* b: send IPI IPI-induced mb |
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* c: smp_mb() |
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* r2 = y |
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* y = 1 |
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* barrier() |
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* r1 = x |
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* |
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* BUG_ON(r1 == 0 && r2 == 0) |
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* |
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* The write to y and load from x by CPU1 are unordered by the hardware, |
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* so it's possible to have "r1 = x" reordered before "y = 1" at any |
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* point after (b). If the memory barrier at (a) is omitted, then "x = 1" |
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* can be reordered after (a) (although not after (c)), so we get r1 == 0 |
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* and r2 == 0. This violates the guarantee that membarrier() is |
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* supposed by provide. |
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* |
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* The timing of the memory barrier at (a) has to ensure that it executes |
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* before the IPI-induced memory barrier on CPU1. |
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* |
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* B) Userspace thread execution before IPI vs membarrier's memory |
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* barrier after completing the IPI |
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* |
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* Userspace variables: |
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* |
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* int x = 0, y = 0; |
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* |
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* The memory barrier at the end of membarrier() on CPU0 is necessary in |
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* order to enforce the guarantee that any writes occurring on CPU1 before |
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* the membarrier() is executed will be visible to any code executing on |
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* CPU0 after the membarrier(): |
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* |
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* CPU0 CPU1 |
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* |
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* x = 1 |
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* barrier() |
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* y = 1 |
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* r2 = y |
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* membarrier(): |
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* a: smp_mb() |
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* b: send IPI IPI-induced mb |
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* c: smp_mb() |
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* r1 = x |
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* BUG_ON(r1 == 0 && r2 == 1) |
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* |
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* The writes to x and y are unordered by the hardware, so it's possible to |
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* have "r2 = 1" even though the write to x doesn't execute until (b). If |
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* the memory barrier at (c) is omitted then "r1 = x" can be reordered |
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* before (b) (although not before (a)), so we get "r1 = 0". This violates |
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* the guarantee that membarrier() is supposed to provide. |
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* |
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* The timing of the memory barrier at (c) has to ensure that it executes |
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* after the IPI-induced memory barrier on CPU1. |
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* |
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* C) Scheduling userspace thread -> kthread -> userspace thread vs membarrier |
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* |
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* CPU0 CPU1 |
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* |
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* membarrier(): |
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* a: smp_mb() |
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* d: switch to kthread (includes mb) |
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* b: read rq->curr->mm == NULL |
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* e: switch to user (includes mb) |
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* c: smp_mb() |
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* |
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* Using the scenario from (A), we can show that (a) needs to be paired |
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* with (e). Using the scenario from (B), we can show that (c) needs to |
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* be paired with (d). |
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* |
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* D) exit_mm vs membarrier |
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* |
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* Two thread groups are created, A and B. Thread group B is created by |
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* issuing clone from group A with flag CLONE_VM set, but not CLONE_THREAD. |
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* Let's assume we have a single thread within each thread group (Thread A |
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* and Thread B). Thread A runs on CPU0, Thread B runs on CPU1. |
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* |
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* CPU0 CPU1 |
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* |
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* membarrier(): |
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* a: smp_mb() |
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* exit_mm(): |
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* d: smp_mb() |
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* e: current->mm = NULL |
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* b: read rq->curr->mm == NULL |
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* c: smp_mb() |
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* |
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* Using scenario (B), we can show that (c) needs to be paired with (d). |
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* |
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* E) kthread_{use,unuse}_mm vs membarrier |
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* |
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* CPU0 CPU1 |
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* |
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* membarrier(): |
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* a: smp_mb() |
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* kthread_unuse_mm() |
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* d: smp_mb() |
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* e: current->mm = NULL |
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* b: read rq->curr->mm == NULL |
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* kthread_use_mm() |
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* f: current->mm = mm |
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* g: smp_mb() |
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* c: smp_mb() |
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* |
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* Using the scenario from (A), we can show that (a) needs to be paired |
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* with (g). Using the scenario from (B), we can show that (c) needs to |
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* be paired with (d). |
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*/ |
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|
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/* |
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* Bitmask made from a "or" of all commands within enum membarrier_cmd, |
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* except MEMBARRIER_CMD_QUERY. |
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*/ |
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#ifdef CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE |
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#define MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK \ |
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(MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE \ |
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| MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE) |
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#else |
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#define MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK 0 |
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#endif |
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|
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#ifdef CONFIG_RSEQ |
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#define MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ_BITMASK \ |
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(MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ \ |
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| MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ_BITMASK) |
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#else |
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#define MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ_BITMASK 0 |
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#endif |
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#define MEMBARRIER_CMD_BITMASK \ |
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(MEMBARRIER_CMD_GLOBAL | MEMBARRIER_CMD_GLOBAL_EXPEDITED \ |
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| MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED \ |
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| MEMBARRIER_CMD_PRIVATE_EXPEDITED \ |
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| MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED \ |
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| MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK) |
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|
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static void ipi_mb(void *info) |
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{ |
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smp_mb(); /* IPIs should be serializing but paranoid. */ |
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} |
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|
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static void ipi_sync_core(void *info) |
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{ |
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/* |
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* The smp_mb() in membarrier after all the IPIs is supposed to |
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* ensure that memory on remote CPUs that occur before the IPI |
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* become visible to membarrier()'s caller -- see scenario B in |
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* the big comment at the top of this file. |
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* |
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* A sync_core() would provide this guarantee, but |
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* sync_core_before_usermode() might end up being deferred until |
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* after membarrier()'s smp_mb(). |
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*/ |
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smp_mb(); /* IPIs should be serializing but paranoid. */ |
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|
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sync_core_before_usermode(); |
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} |
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|
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static void ipi_rseq(void *info) |
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{ |
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/* |
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* Ensure that all stores done by the calling thread are visible |
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* to the current task before the current task resumes. We could |
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* probably optimize this away on most architectures, but by the |
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* time we've already sent an IPI, the cost of the extra smp_mb() |
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* is negligible. |
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*/ |
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smp_mb(); |
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rseq_preempt(current); |
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} |
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|
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static void ipi_sync_rq_state(void *info) |
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{ |
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struct mm_struct *mm = (struct mm_struct *) info; |
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|
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if (current->mm != mm) |
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return; |
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this_cpu_write(runqueues.membarrier_state, |
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atomic_read(&mm->membarrier_state)); |
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/* |
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* Issue a memory barrier after setting |
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* MEMBARRIER_STATE_GLOBAL_EXPEDITED in the current runqueue to |
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* guarantee that no memory access following registration is reordered |
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* before registration. |
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*/ |
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smp_mb(); |
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} |
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|
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void membarrier_exec_mmap(struct mm_struct *mm) |
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{ |
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/* |
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* Issue a memory barrier before clearing membarrier_state to |
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* guarantee that no memory access prior to exec is reordered after |
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* clearing this state. |
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*/ |
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smp_mb(); |
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atomic_set(&mm->membarrier_state, 0); |
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/* |
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* Keep the runqueue membarrier_state in sync with this mm |
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* membarrier_state. |
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*/ |
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this_cpu_write(runqueues.membarrier_state, 0); |
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} |
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|
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void membarrier_update_current_mm(struct mm_struct *next_mm) |
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{ |
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struct rq *rq = this_rq(); |
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int membarrier_state = 0; |
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|
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if (next_mm) |
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membarrier_state = atomic_read(&next_mm->membarrier_state); |
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if (READ_ONCE(rq->membarrier_state) == membarrier_state) |
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return; |
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WRITE_ONCE(rq->membarrier_state, membarrier_state); |
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} |
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|
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static int membarrier_global_expedited(void) |
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{ |
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int cpu; |
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cpumask_var_t tmpmask; |
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|
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if (num_online_cpus() == 1) |
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return 0; |
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|
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/* |
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* Matches memory barriers around rq->curr modification in |
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* scheduler. |
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*/ |
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smp_mb(); /* system call entry is not a mb. */ |
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|
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if (!zalloc_cpumask_var(&tmpmask, GFP_KERNEL)) |
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return -ENOMEM; |
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cpus_read_lock(); |
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rcu_read_lock(); |
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for_each_online_cpu(cpu) { |
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struct task_struct *p; |
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/* |
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* Skipping the current CPU is OK even through we can be |
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* migrated at any point. The current CPU, at the point |
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* where we read raw_smp_processor_id(), is ensured to |
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* be in program order with respect to the caller |
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* thread. Therefore, we can skip this CPU from the |
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* iteration. |
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*/ |
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if (cpu == raw_smp_processor_id()) |
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continue; |
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|
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if (!(READ_ONCE(cpu_rq(cpu)->membarrier_state) & |
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MEMBARRIER_STATE_GLOBAL_EXPEDITED)) |
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continue; |
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|
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/* |
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* Skip the CPU if it runs a kernel thread which is not using |
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* a task mm. |
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*/ |
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p = rcu_dereference(cpu_rq(cpu)->curr); |
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if (!p->mm) |
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continue; |
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__cpumask_set_cpu(cpu, tmpmask); |
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} |
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rcu_read_unlock(); |
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preempt_disable(); |
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smp_call_function_many(tmpmask, ipi_mb, NULL, 1); |
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preempt_enable(); |
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free_cpumask_var(tmpmask); |
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cpus_read_unlock(); |
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/* |
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* Memory barrier on the caller thread _after_ we finished |
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* waiting for the last IPI. Matches memory barriers around |
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* rq->curr modification in scheduler. |
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*/ |
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smp_mb(); /* exit from system call is not a mb */ |
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return 0; |
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} |
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|
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static int membarrier_private_expedited(int flags, int cpu_id) |
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{ |
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cpumask_var_t tmpmask; |
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struct mm_struct *mm = current->mm; |
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smp_call_func_t ipi_func = ipi_mb; |
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if (flags == MEMBARRIER_FLAG_SYNC_CORE) { |
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if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE)) |
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return -EINVAL; |
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if (!(atomic_read(&mm->membarrier_state) & |
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MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY)) |
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return -EPERM; |
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ipi_func = ipi_sync_core; |
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} else if (flags == MEMBARRIER_FLAG_RSEQ) { |
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if (!IS_ENABLED(CONFIG_RSEQ)) |
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return -EINVAL; |
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if (!(atomic_read(&mm->membarrier_state) & |
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MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ_READY)) |
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return -EPERM; |
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ipi_func = ipi_rseq; |
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} else { |
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WARN_ON_ONCE(flags); |
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if (!(atomic_read(&mm->membarrier_state) & |
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MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY)) |
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return -EPERM; |
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} |
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if (flags != MEMBARRIER_FLAG_SYNC_CORE && |
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(atomic_read(&mm->mm_users) == 1 || num_online_cpus() == 1)) |
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return 0; |
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|
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/* |
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* Matches memory barriers around rq->curr modification in |
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* scheduler. |
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*/ |
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smp_mb(); /* system call entry is not a mb. */ |
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|
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if (cpu_id < 0 && !zalloc_cpumask_var(&tmpmask, GFP_KERNEL)) |
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return -ENOMEM; |
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cpus_read_lock(); |
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|
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if (cpu_id >= 0) { |
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struct task_struct *p; |
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|
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if (cpu_id >= nr_cpu_ids || !cpu_online(cpu_id)) |
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goto out; |
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rcu_read_lock(); |
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p = rcu_dereference(cpu_rq(cpu_id)->curr); |
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if (!p || p->mm != mm) { |
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rcu_read_unlock(); |
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goto out; |
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} |
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rcu_read_unlock(); |
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} else { |
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int cpu; |
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rcu_read_lock(); |
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for_each_online_cpu(cpu) { |
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struct task_struct *p; |
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|
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p = rcu_dereference(cpu_rq(cpu)->curr); |
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if (p && p->mm == mm) |
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__cpumask_set_cpu(cpu, tmpmask); |
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} |
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rcu_read_unlock(); |
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} |
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|
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if (cpu_id >= 0) { |
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/* |
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* smp_call_function_single() will call ipi_func() if cpu_id |
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* is the calling CPU. |
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*/ |
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smp_call_function_single(cpu_id, ipi_func, NULL, 1); |
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} else { |
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/* |
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* For regular membarrier, we can save a few cycles by |
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* skipping the current cpu -- we're about to do smp_mb() |
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* below, and if we migrate to a different cpu, this cpu |
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* and the new cpu will execute a full barrier in the |
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* scheduler. |
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* |
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* For SYNC_CORE, we do need a barrier on the current cpu -- |
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* otherwise, if we are migrated and replaced by a different |
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* task in the same mm just before, during, or after |
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* membarrier, we will end up with some thread in the mm |
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* running without a core sync. |
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* |
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* For RSEQ, don't rseq_preempt() the caller. User code |
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* is not supposed to issue syscalls at all from inside an |
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* rseq critical section. |
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*/ |
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if (flags != MEMBARRIER_FLAG_SYNC_CORE) { |
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preempt_disable(); |
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smp_call_function_many(tmpmask, ipi_func, NULL, true); |
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preempt_enable(); |
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} else { |
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on_each_cpu_mask(tmpmask, ipi_func, NULL, true); |
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} |
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} |
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out: |
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if (cpu_id < 0) |
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free_cpumask_var(tmpmask); |
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cpus_read_unlock(); |
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|
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/* |
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* Memory barrier on the caller thread _after_ we finished |
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* waiting for the last IPI. Matches memory barriers around |
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* rq->curr modification in scheduler. |
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*/ |
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smp_mb(); /* exit from system call is not a mb */ |
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|
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return 0; |
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} |
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|
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static int sync_runqueues_membarrier_state(struct mm_struct *mm) |
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{ |
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int membarrier_state = atomic_read(&mm->membarrier_state); |
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cpumask_var_t tmpmask; |
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int cpu; |
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|
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if (atomic_read(&mm->mm_users) == 1 || num_online_cpus() == 1) { |
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this_cpu_write(runqueues.membarrier_state, membarrier_state); |
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|
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/* |
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* For single mm user, we can simply issue a memory barrier |
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* after setting MEMBARRIER_STATE_GLOBAL_EXPEDITED in the |
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* mm and in the current runqueue to guarantee that no memory |
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* access following registration is reordered before |
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* registration. |
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*/ |
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smp_mb(); |
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return 0; |
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} |
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|
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if (!zalloc_cpumask_var(&tmpmask, GFP_KERNEL)) |
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return -ENOMEM; |
|
|
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/* |
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* For mm with multiple users, we need to ensure all future |
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* scheduler executions will observe @mm's new membarrier |
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* state. |
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*/ |
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synchronize_rcu(); |
|
|
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/* |
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* For each cpu runqueue, if the task's mm match @mm, ensure that all |
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* @mm's membarrier state set bits are also set in the runqueue's |
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* membarrier state. This ensures that a runqueue scheduling |
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* between threads which are users of @mm has its membarrier state |
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* updated. |
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*/ |
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cpus_read_lock(); |
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rcu_read_lock(); |
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for_each_online_cpu(cpu) { |
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struct rq *rq = cpu_rq(cpu); |
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struct task_struct *p; |
|
|
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p = rcu_dereference(rq->curr); |
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if (p && p->mm == mm) |
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__cpumask_set_cpu(cpu, tmpmask); |
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} |
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rcu_read_unlock(); |
|
|
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on_each_cpu_mask(tmpmask, ipi_sync_rq_state, mm, true); |
|
|
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free_cpumask_var(tmpmask); |
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cpus_read_unlock(); |
|
|
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return 0; |
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} |
|
|
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static int membarrier_register_global_expedited(void) |
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{ |
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struct task_struct *p = current; |
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struct mm_struct *mm = p->mm; |
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int ret; |
|
|
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if (atomic_read(&mm->membarrier_state) & |
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MEMBARRIER_STATE_GLOBAL_EXPEDITED_READY) |
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return 0; |
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atomic_or(MEMBARRIER_STATE_GLOBAL_EXPEDITED, &mm->membarrier_state); |
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ret = sync_runqueues_membarrier_state(mm); |
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if (ret) |
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return ret; |
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atomic_or(MEMBARRIER_STATE_GLOBAL_EXPEDITED_READY, |
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&mm->membarrier_state); |
|
|
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return 0; |
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} |
|
|
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static int membarrier_register_private_expedited(int flags) |
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{ |
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struct task_struct *p = current; |
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struct mm_struct *mm = p->mm; |
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int ready_state = MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY, |
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set_state = MEMBARRIER_STATE_PRIVATE_EXPEDITED, |
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ret; |
|
|
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if (flags == MEMBARRIER_FLAG_SYNC_CORE) { |
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if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE)) |
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return -EINVAL; |
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ready_state = |
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MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY; |
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} else if (flags == MEMBARRIER_FLAG_RSEQ) { |
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if (!IS_ENABLED(CONFIG_RSEQ)) |
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return -EINVAL; |
|
ready_state = |
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MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ_READY; |
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} else { |
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WARN_ON_ONCE(flags); |
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} |
|
|
|
/* |
|
* We need to consider threads belonging to different thread |
|
* groups, which use the same mm. (CLONE_VM but not |
|
* CLONE_THREAD). |
|
*/ |
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if ((atomic_read(&mm->membarrier_state) & ready_state) == ready_state) |
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return 0; |
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if (flags & MEMBARRIER_FLAG_SYNC_CORE) |
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set_state |= MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE; |
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if (flags & MEMBARRIER_FLAG_RSEQ) |
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set_state |= MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ; |
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atomic_or(set_state, &mm->membarrier_state); |
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ret = sync_runqueues_membarrier_state(mm); |
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if (ret) |
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return ret; |
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atomic_or(ready_state, &mm->membarrier_state); |
|
|
|
return 0; |
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} |
|
|
|
/** |
|
* sys_membarrier - issue memory barriers on a set of threads |
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* @cmd: Takes command values defined in enum membarrier_cmd. |
|
* @flags: Currently needs to be 0 for all commands other than |
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* MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ: in the latter |
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* case it can be MEMBARRIER_CMD_FLAG_CPU, indicating that @cpu_id |
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* contains the CPU on which to interrupt (= restart) |
|
* the RSEQ critical section. |
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* @cpu_id: if @flags == MEMBARRIER_CMD_FLAG_CPU, indicates the cpu on which |
|
* RSEQ CS should be interrupted (@cmd must be |
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* MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ). |
|
* |
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* If this system call is not implemented, -ENOSYS is returned. If the |
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* command specified does not exist, not available on the running |
|
* kernel, or if the command argument is invalid, this system call |
|
* returns -EINVAL. For a given command, with flags argument set to 0, |
|
* if this system call returns -ENOSYS or -EINVAL, it is guaranteed to |
|
* always return the same value until reboot. In addition, it can return |
|
* -ENOMEM if there is not enough memory available to perform the system |
|
* call. |
|
* |
|
* All memory accesses performed in program order from each targeted thread |
|
* is guaranteed to be ordered with respect to sys_membarrier(). If we use |
|
* the semantic "barrier()" to represent a compiler barrier forcing memory |
|
* accesses to be performed in program order across the barrier, and |
|
* smp_mb() to represent explicit memory barriers forcing full memory |
|
* ordering across the barrier, we have the following ordering table for |
|
* each pair of barrier(), sys_membarrier() and smp_mb(): |
|
* |
|
* The pair ordering is detailed as (O: ordered, X: not ordered): |
|
* |
|
* barrier() smp_mb() sys_membarrier() |
|
* barrier() X X O |
|
* smp_mb() X O O |
|
* sys_membarrier() O O O |
|
*/ |
|
SYSCALL_DEFINE3(membarrier, int, cmd, unsigned int, flags, int, cpu_id) |
|
{ |
|
switch (cmd) { |
|
case MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ: |
|
if (unlikely(flags && flags != MEMBARRIER_CMD_FLAG_CPU)) |
|
return -EINVAL; |
|
break; |
|
default: |
|
if (unlikely(flags)) |
|
return -EINVAL; |
|
} |
|
|
|
if (!(flags & MEMBARRIER_CMD_FLAG_CPU)) |
|
cpu_id = -1; |
|
|
|
switch (cmd) { |
|
case MEMBARRIER_CMD_QUERY: |
|
{ |
|
int cmd_mask = MEMBARRIER_CMD_BITMASK; |
|
|
|
if (tick_nohz_full_enabled()) |
|
cmd_mask &= ~MEMBARRIER_CMD_GLOBAL; |
|
return cmd_mask; |
|
} |
|
case MEMBARRIER_CMD_GLOBAL: |
|
/* MEMBARRIER_CMD_GLOBAL is not compatible with nohz_full. */ |
|
if (tick_nohz_full_enabled()) |
|
return -EINVAL; |
|
if (num_online_cpus() > 1) |
|
synchronize_rcu(); |
|
return 0; |
|
case MEMBARRIER_CMD_GLOBAL_EXPEDITED: |
|
return membarrier_global_expedited(); |
|
case MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED: |
|
return membarrier_register_global_expedited(); |
|
case MEMBARRIER_CMD_PRIVATE_EXPEDITED: |
|
return membarrier_private_expedited(0, cpu_id); |
|
case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED: |
|
return membarrier_register_private_expedited(0); |
|
case MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE: |
|
return membarrier_private_expedited(MEMBARRIER_FLAG_SYNC_CORE, cpu_id); |
|
case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE: |
|
return membarrier_register_private_expedited(MEMBARRIER_FLAG_SYNC_CORE); |
|
case MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ: |
|
return membarrier_private_expedited(MEMBARRIER_FLAG_RSEQ, cpu_id); |
|
case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ: |
|
return membarrier_register_private_expedited(MEMBARRIER_FLAG_RSEQ); |
|
default: |
|
return -EINVAL; |
|
} |
|
}
|
|
|