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locking/rwbase: Take care of ordering guarantee for fastpath reader

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Readers of rwbase can lock and unlock without taking any inner lock, if
that happens, we need the ordering provided by atomic operations to
satisfy the ordering semantics of lock/unlock. Without that, considering
the follow case:

	{ X = 0 initially }

	CPU 0			CPU 1
	=====			=====
				rt_write_lock();
				X = 1
				rt_write_unlock():
				  atomic_add(READER_BIAS - WRITER_BIAS, ->readers);
				  // ->readers is READER_BIAS.
	rt_read_lock():
	  if ((r = atomic_read(->readers)) < 0) // True
	    atomic_try_cmpxchg(->readers, r, r + 1); // succeed.
	  <acquire the read lock via fast path>

	r1 = X;	// r1 may be 0, because nothing prevent the reordering
	        // of "X=1" and atomic_add() on CPU 1.

Therefore audit every usage of atomic operations that may happen in a
fast path, and add necessary barriers.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20210909110203.953991276@infradead.org
This commit is contained in:
Boqun Feng 2021-09-09 12:59:19 +02:00 committed by Peter Zijlstra
parent 616be87eac
commit 81121524f1
1 changed files with 19 additions and 2 deletions
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21
kernel/locking/rwbase_rt.c
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@ -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 should thus provide _acquire()
* and _release() (or stronger).
*
* 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);
raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
rwbase_rtmutex_unlock(rtm);
}
@ -201,7 +214,11 @@ static inline bool __rwbase_write_trylock(struct rwbase_rt *rwb)
/* Can do without CAS because we're serialized by wait_lock. */
lockdep_assert_held(&rwb->rtmutex.wait_lock);
if (!atomic_read(&rwb->readers)) {
/*
* _acquire is needed in case the reader is in the fast path, pairing
* with rwbase_read_unlock(), provides ACQUIRE.
*/
if (!atomic_read_acquire(&rwb->readers)) {
atomic_set(&rwb->readers, WRITER_BIAS);
return 1;
}