Messages in this thread Patch in this message | | | Date | Thu, 2 Sep 2021 13:55:29 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC] locking: rwbase: Take care of ordering guarantee for fastpath reader |
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On Wed, Sep 01, 2021 at 11:06:27PM +0800, Boqun Feng wrote:
> Sorry I'm late for the party of PREEMPT_RT lock review. Just want to > point the problem with this patch. Not even compile test, but show the > idea and check if I'm missing something subtle.
No worries, glad you could have a look. I think you're right and we missed this.
> diff --git a/kernel/locking/rwbase_rt.c b/kernel/locking/rwbase_rt.c > index 4ba15088e640..a1886fd8bde6 100644 > --- a/kernel/locking/rwbase_rt.c > +++ b/kernel/locking/rwbase_rt.c > @@ -41,6 +41,12 @@ > * The risk of writer starvation is there, but the pathological use cases > * which trigger it are not necessarily the typical RT workloads. > * > + * Fast-path orderings: > + * The lock/unlock of readers can run in fast paths: lock and unlock are only > + * atomic ops, and there is no inner lock to provide ACQUIRE and RELEASE > + * semantics of rwbase_rt. Atomic ops then should be stronger than _acquire() > + * and _release() to provide necessary ordering guarantee. > + * > * Common code shared between RT rw_semaphore and rwlock > */ > > @@ -53,6 +59,7 @@ static __always_inline int rwbase_read_trylock(struct rwbase_rt *rwb) > * set. > */ > for (r = atomic_read(&rwb->readers); r < 0;) { > + /* Fully-ordered if cmpxchg() succeeds, provides ACQUIRE */ > if (likely(atomic_try_cmpxchg(&rwb->readers, &r, r + 1))) > return 1; > } > @@ -162,6 +169,8 @@ static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb, > /* > * rwb->readers can only hit 0 when a writer is waiting for the > * active readers to leave the critical section. > + * > + * dec_and_test() is fully ordered, provides RELEASE. > */ > if (unlikely(atomic_dec_and_test(&rwb->readers))) > __rwbase_read_unlock(rwb, state); > @@ -172,7 +181,11 @@ static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias, > { > struct rt_mutex_base *rtm = &rwb->rtmutex; > > - atomic_add(READER_BIAS - bias, &rwb->readers); > + /* > + * _release() is needed in case that reader is in fast path, pairing > + * with atomic_try_cmpxchg() in rwbase_read_trylock(), provides RELEASE > + */ > + (void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers);
Very narrow race with the unlock below, but yes agreed.
> raw_spin_unlock_irqrestore(&rtm->wait_lock, flags); > rwbase_rtmutex_unlock(rtm); > } > @@ -216,8 +229,14 @@ static int __sched rwbase_write_lock(struct rwbase_rt *rwb, > */ > rwbase_set_and_save_current_state(state); > > - /* Block until all readers have left the critical section. */ > - for (; atomic_read(&rwb->readers);) { > + /* > + * Block until all readers have left the critical section. > + * > + * _acqurie() is needed in case that the reader side runs in the fast > + * path, pairing with the atomic_dec_and_test() in rwbase_read_unlock(), > + * provides ACQUIRE. > + */ > + for (; atomic_read_acquire(&rwb->readers);) { > /* Optimized out for rwlocks */ > if (rwbase_signal_pending_state(state, current)) { > __set_current_state(TASK_RUNNING);
I think we can restructure things to avoid this one, but yes. Suppose we do:
readers = atomic_sub_return_relaxed(READER_BIAS, &rwb->readers);
/* * These two provide either an smp_mb() or an UNLOCK+LOCK * ordering, either is strong enough to provide ACQUIRE order * for the above load of @readers. */ rwbase_set_and_save_current_state(state); raw_spin_lock_irqsave(&rtm->wait_lock, flags);
while (readers) { ... readers = atomic_read(&rwb->readers); if (readers) rwbase_schedule(); ... }
> @@ -229,6 +248,9 @@ static int __sched rwbase_write_lock(struct rwbase_rt *rwb, > /* > * Schedule and wait for the readers to leave the critical > * section. The last reader leaving it wakes the waiter. > + * > + * _acquire() is not needed, because we can rely on the smp_mb() > + * in set_current_state() to provide ACQUIRE. > */ > if (atomic_read(&rwb->readers) != 0) > rwbase_schedule(); > @@ -253,7 +275,11 @@ static inline int rwbase_write_trylock(struct rwbase_rt *rwb) > atomic_sub(READER_BIAS, &rwb->readers); > > raw_spin_lock_irqsave(&rtm->wait_lock, flags); > - if (!atomic_read(&rwb->readers)) { > + /* > + * _acquire() is needed in case reader is in the fast path, pairing with > + * rwbase_read_unlock(), provides ACQUIRE. > + */ > + if (!atomic_read_acquire(&rwb->readers)) {
Moo; the alternative is using dec_and_lock instead of dec_and_test, but that's not going to be worth it.
> atomic_set(&rwb->readers, WRITER_BIAS); > raw_spin_unlock_irqrestore(&rtm->wait_lock, flags); > return 1; > -- > 2.32.0 >
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