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Subject[PATCH v5.2 2/2] lcoking/rwsem: Add missing ACQUIRE to read_slowpath sleep loop
Date
From: Peter Zijlstra <peterz@infradead.org>

[ Upstream commit 99143f82a255e7f054bead8443462fae76dd829e ]

While reviewing another read_slowpath patch, both Will and I noticed
another missing ACQUIRE, namely:

X = 0;

CPU0 CPU1

rwsem_down_read()
for (;;) {
set_current_state(TASK_UNINTERRUPTIBLE);

X = 1;
rwsem_up_write();
rwsem_mark_wake()
atomic_long_add(adjustment, &sem->count);
smp_store_release(&waiter->task, NULL);

if (!waiter.task)
break;

...
}

r = X;

Allows 'r == 0'.

Reported-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reported-by: Will Deacon <will@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

This is a backport for the v5.2 stable tree. There were multiple reports
of this issue being hit.

Given that there were a few changes to the code around this, I'd
appreciate an ack before pulling it in.

kernel/locking/rwsem-xadd.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 397dedc58432d..385ebcfc31a6d 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -485,8 +485,10 @@ __rwsem_down_read_failed_common(struct rw_semaphore *sem, int state)
/* wait to be given the lock */
while (true) {
set_current_state(state);
- if (!waiter.task)
+ if (!smp_load_acquire(&waiter.task)) {
+ /* Orders against rwsem_mark_wake()'s smp_store_release() */
break;
+ }
if (signal_pending_state(state, current)) {
raw_spin_lock_irq(&sem->wait_lock);
if (waiter.task)
--
2.20.1
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