299 lines
8.2 KiB
C
299 lines
8.2 KiB
C
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// 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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