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206 lines
6.2 KiB
206 lines
6.2 KiB
// SPDX-License-Identifier: GPL-2.0+ |
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/* |
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* RCU-based infrastructure for lightweight reader-writer locking |
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* |
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* Copyright (c) 2015, Red Hat, Inc. |
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* |
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* Author: Oleg Nesterov <[email protected]> |
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*/ |
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#include <linux/rcu_sync.h> |
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#include <linux/sched.h> |
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enum { GP_IDLE = 0, GP_ENTER, GP_PASSED, GP_EXIT, GP_REPLAY }; |
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#define rss_lock gp_wait.lock |
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/** |
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* rcu_sync_init() - Initialize an rcu_sync structure |
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* @rsp: Pointer to rcu_sync structure to be initialized |
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*/ |
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void rcu_sync_init(struct rcu_sync *rsp) |
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{ |
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memset(rsp, 0, sizeof(*rsp)); |
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init_waitqueue_head(&rsp->gp_wait); |
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} |
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/** |
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* rcu_sync_enter_start - Force readers onto slow path for multiple updates |
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* @rsp: Pointer to rcu_sync structure to use for synchronization |
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* |
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* Must be called after rcu_sync_init() and before first use. |
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* |
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* Ensures rcu_sync_is_idle() returns false and rcu_sync_{enter,exit}() |
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* pairs turn into NO-OPs. |
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*/ |
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void rcu_sync_enter_start(struct rcu_sync *rsp) |
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{ |
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rsp->gp_count++; |
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rsp->gp_state = GP_PASSED; |
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} |
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static void rcu_sync_func(struct rcu_head *rhp); |
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static void rcu_sync_call(struct rcu_sync *rsp) |
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{ |
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call_rcu(&rsp->cb_head, rcu_sync_func); |
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} |
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/** |
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* rcu_sync_func() - Callback function managing reader access to fastpath |
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* @rhp: Pointer to rcu_head in rcu_sync structure to use for synchronization |
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* |
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* This function is passed to call_rcu() function by rcu_sync_enter() and |
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* rcu_sync_exit(), so that it is invoked after a grace period following the |
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* that invocation of enter/exit. |
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* |
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* If it is called by rcu_sync_enter() it signals that all the readers were |
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* switched onto slow path. |
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* |
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* If it is called by rcu_sync_exit() it takes action based on events that |
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* have taken place in the meantime, so that closely spaced rcu_sync_enter() |
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* and rcu_sync_exit() pairs need not wait for a grace period. |
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* |
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* If another rcu_sync_enter() is invoked before the grace period |
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* ended, reset state to allow the next rcu_sync_exit() to let the |
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* readers back onto their fastpaths (after a grace period). If both |
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* another rcu_sync_enter() and its matching rcu_sync_exit() are invoked |
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* before the grace period ended, re-invoke call_rcu() on behalf of that |
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* rcu_sync_exit(). Otherwise, set all state back to idle so that readers |
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* can again use their fastpaths. |
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*/ |
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static void rcu_sync_func(struct rcu_head *rhp) |
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{ |
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struct rcu_sync *rsp = container_of(rhp, struct rcu_sync, cb_head); |
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unsigned long flags; |
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WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE); |
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WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED); |
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spin_lock_irqsave(&rsp->rss_lock, flags); |
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if (rsp->gp_count) { |
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/* |
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* We're at least a GP after the GP_IDLE->GP_ENTER transition. |
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*/ |
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WRITE_ONCE(rsp->gp_state, GP_PASSED); |
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wake_up_locked(&rsp->gp_wait); |
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} else if (rsp->gp_state == GP_REPLAY) { |
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/* |
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* A new rcu_sync_exit() has happened; requeue the callback to |
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* catch a later GP. |
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*/ |
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WRITE_ONCE(rsp->gp_state, GP_EXIT); |
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rcu_sync_call(rsp); |
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} else { |
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/* |
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* We're at least a GP after the last rcu_sync_exit(); everybody |
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* will now have observed the write side critical section. |
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* Let 'em rip! |
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*/ |
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WRITE_ONCE(rsp->gp_state, GP_IDLE); |
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} |
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spin_unlock_irqrestore(&rsp->rss_lock, flags); |
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} |
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/** |
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* rcu_sync_enter() - Force readers onto slowpath |
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* @rsp: Pointer to rcu_sync structure to use for synchronization |
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* |
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* This function is used by updaters who need readers to make use of |
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* a slowpath during the update. After this function returns, all |
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* subsequent calls to rcu_sync_is_idle() will return false, which |
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* tells readers to stay off their fastpaths. A later call to |
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* rcu_sync_exit() re-enables reader slowpaths. |
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* |
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* When called in isolation, rcu_sync_enter() must wait for a grace |
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* period, however, closely spaced calls to rcu_sync_enter() can |
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* optimize away the grace-period wait via a state machine implemented |
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* by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func(). |
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*/ |
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void rcu_sync_enter(struct rcu_sync *rsp) |
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{ |
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int gp_state; |
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spin_lock_irq(&rsp->rss_lock); |
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gp_state = rsp->gp_state; |
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if (gp_state == GP_IDLE) { |
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WRITE_ONCE(rsp->gp_state, GP_ENTER); |
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WARN_ON_ONCE(rsp->gp_count); |
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/* |
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* Note that we could simply do rcu_sync_call(rsp) here and |
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* avoid the "if (gp_state == GP_IDLE)" block below. |
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* |
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* However, synchronize_rcu() can be faster if rcu_expedited |
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* or rcu_blocking_is_gp() is true. |
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* |
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* Another reason is that we can't wait for rcu callback if |
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* we are called at early boot time but this shouldn't happen. |
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*/ |
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} |
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rsp->gp_count++; |
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spin_unlock_irq(&rsp->rss_lock); |
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if (gp_state == GP_IDLE) { |
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/* |
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* See the comment above, this simply does the "synchronous" |
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* call_rcu(rcu_sync_func) which does GP_ENTER -> GP_PASSED. |
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*/ |
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synchronize_rcu(); |
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rcu_sync_func(&rsp->cb_head); |
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/* Not really needed, wait_event() would see GP_PASSED. */ |
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return; |
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} |
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wait_event(rsp->gp_wait, READ_ONCE(rsp->gp_state) >= GP_PASSED); |
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} |
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/** |
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* rcu_sync_exit() - Allow readers back onto fast path after grace period |
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* @rsp: Pointer to rcu_sync structure to use for synchronization |
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* |
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* This function is used by updaters who have completed, and can therefore |
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* now allow readers to make use of their fastpaths after a grace period |
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* has elapsed. After this grace period has completed, all subsequent |
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* calls to rcu_sync_is_idle() will return true, which tells readers that |
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* they can once again use their fastpaths. |
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*/ |
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void rcu_sync_exit(struct rcu_sync *rsp) |
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{ |
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WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE); |
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WARN_ON_ONCE(READ_ONCE(rsp->gp_count) == 0); |
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spin_lock_irq(&rsp->rss_lock); |
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if (!--rsp->gp_count) { |
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if (rsp->gp_state == GP_PASSED) { |
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WRITE_ONCE(rsp->gp_state, GP_EXIT); |
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rcu_sync_call(rsp); |
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} else if (rsp->gp_state == GP_EXIT) { |
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WRITE_ONCE(rsp->gp_state, GP_REPLAY); |
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} |
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} |
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spin_unlock_irq(&rsp->rss_lock); |
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} |
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/** |
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* rcu_sync_dtor() - Clean up an rcu_sync structure |
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* @rsp: Pointer to rcu_sync structure to be cleaned up |
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*/ |
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void rcu_sync_dtor(struct rcu_sync *rsp) |
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{ |
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int gp_state; |
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WARN_ON_ONCE(READ_ONCE(rsp->gp_count)); |
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WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED); |
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spin_lock_irq(&rsp->rss_lock); |
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if (rsp->gp_state == GP_REPLAY) |
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WRITE_ONCE(rsp->gp_state, GP_EXIT); |
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gp_state = rsp->gp_state; |
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spin_unlock_irq(&rsp->rss_lock); |
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if (gp_state != GP_IDLE) { |
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rcu_barrier(); |
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WARN_ON_ONCE(rsp->gp_state != GP_IDLE); |
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} |
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}
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