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298 lines
8.2 KiB
298 lines
8.2 KiB
// SPDX-License-Identifier: GPL-2.0-only |
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/* |
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* RT-specific reader/writer semaphores and reader/writer locks |
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* |
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* down_write/write_lock() |
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* 1) Lock rtmutex |
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* 2) Remove the reader BIAS to force readers into the slow path |
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* 3) Wait until all readers have left the critical section |
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* 4) Mark it write locked |
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* |
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* up_write/write_unlock() |
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* 1) Remove the write locked marker |
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* 2) Set the reader BIAS, so readers can use the fast path again |
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* 3) Unlock rtmutex, to release blocked readers |
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* |
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* down_read/read_lock() |
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* 1) Try fast path acquisition (reader BIAS is set) |
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* 2) Take tmutex::wait_lock, which protects the writelocked flag |
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* 3) If !writelocked, acquire it for read |
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* 4) If writelocked, block on tmutex |
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* 5) unlock rtmutex, goto 1) |
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* |
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* up_read/read_unlock() |
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* 1) Try fast path release (reader count != 1) |
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* 2) Wake the writer waiting in down_write()/write_lock() #3 |
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* |
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* down_read/read_lock()#3 has the consequence, that rw semaphores and rw |
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* locks on RT are not writer fair, but writers, which should be avoided in |
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* RT tasks (think mmap_sem), are subject to the rtmutex priority/DL |
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* inheritance mechanism. |
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* |
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* It's possible to make the rw primitives writer fair by keeping a list of |
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* active readers. A blocked writer would force all newly incoming readers |
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* to block on the rtmutex, but the rtmutex would have to be proxy locked |
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* for one reader after the other. We can't use multi-reader inheritance |
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* because there is no way to support that with SCHED_DEADLINE. |
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* Implementing the one by one reader boosting/handover mechanism is a |
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* major surgery for a very dubious value. |
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* |
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* The risk of writer starvation is there, but the pathological use cases |
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* which trigger it are not necessarily the typical RT workloads. |
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* |
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* Fast-path orderings: |
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* The lock/unlock of readers can run in fast paths: lock and unlock are only |
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* atomic ops, and there is no inner lock to provide ACQUIRE and RELEASE |
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* semantics of rwbase_rt. Atomic ops should thus provide _acquire() |
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* and _release() (or stronger). |
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* |
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* Common code shared between RT rw_semaphore and rwlock |
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*/ |
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static __always_inline int rwbase_read_trylock(struct rwbase_rt *rwb) |
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{ |
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int r; |
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/* |
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* Increment reader count, if sem->readers < 0, i.e. READER_BIAS is |
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* set. |
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*/ |
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for (r = atomic_read(&rwb->readers); r < 0;) { |
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if (likely(atomic_try_cmpxchg_acquire(&rwb->readers, &r, r + 1))) |
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return 1; |
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} |
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return 0; |
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} |
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static int __sched __rwbase_read_lock(struct rwbase_rt *rwb, |
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unsigned int state) |
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{ |
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struct rt_mutex_base *rtm = &rwb->rtmutex; |
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int ret; |
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raw_spin_lock_irq(&rtm->wait_lock); |
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/* |
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* Allow readers, as long as the writer has not completely |
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* acquired the semaphore for write. |
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*/ |
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if (atomic_read(&rwb->readers) != WRITER_BIAS) { |
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atomic_inc(&rwb->readers); |
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raw_spin_unlock_irq(&rtm->wait_lock); |
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return 0; |
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} |
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/* |
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* Call into the slow lock path with the rtmutex->wait_lock |
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* held, so this can't result in the following race: |
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* |
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* Reader1 Reader2 Writer |
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* down_read() |
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* down_write() |
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* rtmutex_lock(m) |
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* wait() |
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* down_read() |
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* unlock(m->wait_lock) |
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* up_read() |
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* wake(Writer) |
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* lock(m->wait_lock) |
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* sem->writelocked=true |
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* unlock(m->wait_lock) |
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* |
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* up_write() |
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* sem->writelocked=false |
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* rtmutex_unlock(m) |
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* down_read() |
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* down_write() |
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* rtmutex_lock(m) |
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* wait() |
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* rtmutex_lock(m) |
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* |
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* That would put Reader1 behind the writer waiting on |
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* Reader2 to call up_read(), which might be unbound. |
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*/ |
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trace_contention_begin(rwb, LCB_F_RT | LCB_F_READ); |
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/* |
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* For rwlocks this returns 0 unconditionally, so the below |
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* !ret conditionals are optimized out. |
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*/ |
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ret = rwbase_rtmutex_slowlock_locked(rtm, state); |
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/* |
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* On success the rtmutex is held, so there can't be a writer |
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* active. Increment the reader count and immediately drop the |
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* rtmutex again. |
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* |
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* rtmutex->wait_lock has to be unlocked in any case of course. |
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*/ |
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if (!ret) |
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atomic_inc(&rwb->readers); |
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raw_spin_unlock_irq(&rtm->wait_lock); |
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if (!ret) |
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rwbase_rtmutex_unlock(rtm); |
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trace_contention_end(rwb, ret); |
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return ret; |
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} |
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static __always_inline int rwbase_read_lock(struct rwbase_rt *rwb, |
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unsigned int state) |
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{ |
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if (rwbase_read_trylock(rwb)) |
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return 0; |
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return __rwbase_read_lock(rwb, state); |
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} |
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static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb, |
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unsigned int state) |
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{ |
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struct rt_mutex_base *rtm = &rwb->rtmutex; |
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struct task_struct *owner; |
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DEFINE_RT_WAKE_Q(wqh); |
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raw_spin_lock_irq(&rtm->wait_lock); |
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/* |
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* Wake the writer, i.e. the rtmutex owner. It might release the |
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* rtmutex concurrently in the fast path (due to a signal), but to |
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* clean up rwb->readers it needs to acquire rtm->wait_lock. The |
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* worst case which can happen is a spurious wakeup. |
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*/ |
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owner = rt_mutex_owner(rtm); |
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if (owner) |
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rt_mutex_wake_q_add_task(&wqh, owner, state); |
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/* Pairs with the preempt_enable in rt_mutex_wake_up_q() */ |
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preempt_disable(); |
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raw_spin_unlock_irq(&rtm->wait_lock); |
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rt_mutex_wake_up_q(&wqh); |
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} |
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static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb, |
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unsigned int state) |
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{ |
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/* |
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* rwb->readers can only hit 0 when a writer is waiting for the |
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* active readers to leave the critical section. |
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* |
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* dec_and_test() is fully ordered, provides RELEASE. |
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*/ |
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if (unlikely(atomic_dec_and_test(&rwb->readers))) |
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__rwbase_read_unlock(rwb, state); |
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} |
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static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias, |
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unsigned long flags) |
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{ |
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struct rt_mutex_base *rtm = &rwb->rtmutex; |
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/* |
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* _release() is needed in case that reader is in fast path, pairing |
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* with atomic_try_cmpxchg_acquire() in rwbase_read_trylock(). |
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*/ |
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(void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers); |
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raw_spin_unlock_irqrestore(&rtm->wait_lock, flags); |
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rwbase_rtmutex_unlock(rtm); |
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} |
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static inline void rwbase_write_unlock(struct rwbase_rt *rwb) |
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{ |
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struct rt_mutex_base *rtm = &rwb->rtmutex; |
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unsigned long flags; |
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raw_spin_lock_irqsave(&rtm->wait_lock, flags); |
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__rwbase_write_unlock(rwb, WRITER_BIAS, flags); |
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} |
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static inline void rwbase_write_downgrade(struct rwbase_rt *rwb) |
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{ |
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struct rt_mutex_base *rtm = &rwb->rtmutex; |
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unsigned long flags; |
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raw_spin_lock_irqsave(&rtm->wait_lock, flags); |
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/* Release it and account current as reader */ |
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__rwbase_write_unlock(rwb, WRITER_BIAS - 1, flags); |
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} |
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static inline bool __rwbase_write_trylock(struct rwbase_rt *rwb) |
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{ |
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/* Can do without CAS because we're serialized by wait_lock. */ |
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lockdep_assert_held(&rwb->rtmutex.wait_lock); |
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/* |
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* _acquire is needed in case the reader is in the fast path, pairing |
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* with rwbase_read_unlock(), provides ACQUIRE. |
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*/ |
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if (!atomic_read_acquire(&rwb->readers)) { |
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atomic_set(&rwb->readers, WRITER_BIAS); |
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return 1; |
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} |
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return 0; |
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} |
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static int __sched rwbase_write_lock(struct rwbase_rt *rwb, |
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unsigned int state) |
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{ |
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struct rt_mutex_base *rtm = &rwb->rtmutex; |
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unsigned long flags; |
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/* Take the rtmutex as a first step */ |
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if (rwbase_rtmutex_lock_state(rtm, state)) |
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return -EINTR; |
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/* Force readers into slow path */ |
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atomic_sub(READER_BIAS, &rwb->readers); |
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raw_spin_lock_irqsave(&rtm->wait_lock, flags); |
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if (__rwbase_write_trylock(rwb)) |
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goto out_unlock; |
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rwbase_set_and_save_current_state(state); |
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trace_contention_begin(rwb, LCB_F_RT | LCB_F_WRITE); |
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for (;;) { |
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/* Optimized out for rwlocks */ |
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if (rwbase_signal_pending_state(state, current)) { |
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rwbase_restore_current_state(); |
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__rwbase_write_unlock(rwb, 0, flags); |
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trace_contention_end(rwb, -EINTR); |
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return -EINTR; |
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} |
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if (__rwbase_write_trylock(rwb)) |
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break; |
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raw_spin_unlock_irqrestore(&rtm->wait_lock, flags); |
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rwbase_schedule(); |
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raw_spin_lock_irqsave(&rtm->wait_lock, flags); |
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set_current_state(state); |
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} |
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rwbase_restore_current_state(); |
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trace_contention_end(rwb, 0); |
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out_unlock: |
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raw_spin_unlock_irqrestore(&rtm->wait_lock, flags); |
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return 0; |
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} |
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static inline int rwbase_write_trylock(struct rwbase_rt *rwb) |
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{ |
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struct rt_mutex_base *rtm = &rwb->rtmutex; |
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unsigned long flags; |
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if (!rwbase_rtmutex_trylock(rtm)) |
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return 0; |
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atomic_sub(READER_BIAS, &rwb->readers); |
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raw_spin_lock_irqsave(&rtm->wait_lock, flags); |
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if (__rwbase_write_trylock(rwb)) { |
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raw_spin_unlock_irqrestore(&rtm->wait_lock, flags); |
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return 1; |
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} |
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__rwbase_write_unlock(rwb, 0, flags); |
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return 0; |
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}
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