3.16.78-rc1 review patch. If anyone has any objections, please let me know.
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From: Peter Zijlstra peterz@infradead.org
commit 952041a8639a7a3a73a2b6573cb8aa8518bc39f8 upstream.
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: Ben Hutchings ben@decadent.org.uk --- drivers/tty/tty_ldsem.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-)
--- a/drivers/tty/tty_ldsem.c +++ b/drivers/tty/tty_ldsem.c @@ -137,8 +137,7 @@ static void __ldsem_wake_readers(struct
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 *se for (;;) { set_task_state(tsk, TASK_UNINTERRUPTIBLE);
- if (!waiter.task) + if (!smp_load_acquire(&waiter.task)) break; if (!timeout) break;