| From | Greg Kroah-Hartman <> | Subject | [PATCH 4.4 22/78] tty/ldsem, locking/rwsem: Add missing ACQUIRE to read_failed sleep loop | Date | Thu, 22 Aug 2019 10:18:26 -0700 |
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[ 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 34234c2338511..656c2ade6a434 100644 --- a/drivers/tty/tty_ldsem.c +++ b/drivers/tty/tty_ldsem.c @@ -137,8 +137,7 @@ 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); } @@ -234,7 +233,7 @@ down_read_failed(struct ld_semaphore *sem, long count, long timeout) for (;;) { set_task_state(tsk, TASK_UNINTERRUPTIBLE); - if (!waiter.task) + if (!smp_load_acquire(&waiter.task)) break; if (!timeout) break; -- 2.20.1
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