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Subject[PATCH AUTOSEL 5.2 70/76] tty/ldsem, locking/rwsem: Add missing ACQUIRE to read_failed sleep loop
Date
From: Peter Zijlstra <peterz@infradead.org>

[ Upstream commit 952041a8639a7a3a73a2b6573cb8aa8518bc39f8 ]

While reviewing rwsem down_slowpath, Will noticed ldsem had a copy of
a bug we just found for rwsem.

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: 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 Hurley <peter@hurleysoftware.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 4898e640caf0 ("tty: Add timed, writer-prioritized rw semaphore")
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/tty/tty_ldsem.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/tty_ldsem.c b/drivers/tty/tty_ldsem.c
index 717292c1c0dfc..60ff236a3d63d 100644
--- a/drivers/tty/tty_ldsem.c
+++ b/drivers/tty/tty_ldsem.c
@@ -93,8 +93,7 @@ static void __ldsem_wake_readers(struct ld_semaphore *sem)

list_for_each_entry_safe(waiter, next, &sem->read_wait, list) {
tsk = waiter->task;
- smp_mb();
- waiter->task = NULL;
+ smp_store_release(&waiter->task, NULL);
wake_up_process(tsk);
put_task_struct(tsk);
}
@@ -194,7 +193,7 @@ down_read_failed(struct ld_semaphore *sem, long count, long timeout)
for (;;) {
set_current_state(TASK_UNINTERRUPTIBLE);

- if (!waiter.task)
+ if (!smp_load_acquire(&waiter.task))
break;
if (!timeout)
break;
--
2.20.1
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