From: Corey Minyard Date: Thu, 9 May 2019 14:33:20 -0500 Subject: [PATCH 264/351] sched/completion: Fix a lockup in wait_for_completion() Origin: https://git.kernel.org/cgit/linux/kernel/git/rt/linux-stable-rt.git/commit?id=8d147c6201655ace488f41a1c271d3da10e48853 Consider following race: T0 T1 T2 wait_for_completion() do_wait_for_common() __prepare_to_swait() schedule() complete() x->done++ (0 -> 1) raw_spin_lock_irqsave() swake_up_locked() wait_for_completion() wake_up_process(T0) list_del_init() raw_spin_unlock_irqrestore() raw_spin_lock_irq(&x->wait.lock) raw_spin_lock_irq(&x->wait.lock) x->done != UINT_MAX, 1 -> 0 raw_spin_unlock_irq(&x->wait.lock) return 1 while (!x->done && timeout), continue loop, not enqueued on &x->wait Basically, the problem is that the original wait queues used in completions did not remove the item from the queue in the wakeup function, but swake_up_locked() does. Fix it by adding the thread to the wait queue inside the do loop. The design of swait detects if it is already in the list and doesn't do the list add again. Cc: stable-rt@vger.kernel.org Fixes: a04ff6b4ec4ee7e ("completion: Use simple wait queues") Signed-off-by: Corey Minyard Acked-by: Steven Rostedt (VMware) Signed-off-by: Steven Rostedt (VMware) [bigeasy: shorten commit message ] Signed-off-by: Sebastian Andrzej Siewior --- kernel/sched/completion.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c index 755a58084978..49c14137988e 100644 --- a/kernel/sched/completion.c +++ b/kernel/sched/completion.c @@ -72,12 +72,12 @@ do_wait_for_common(struct completion *x, if (!x->done) { DECLARE_SWAITQUEUE(wait); - __prepare_to_swait(&x->wait, &wait); do { if (signal_pending_state(state, current)) { timeout = -ERESTARTSYS; break; } + __prepare_to_swait(&x->wait, &wait); __set_current_state(state); raw_spin_unlock_irq(&x->wait.lock); timeout = action(timeout);