Re: [PATCH RT v2] Fix a lockup in wait_for_completion() and friends

[Steven Rostedt]

On Thu,  9 May 2019 14:33:20 -0500
minyard@xxxxxxx wrote:

> From: Corey Minyard <cminyard@xxxxxxxxxx>
> 
> The function call do_wait_for_common() has a race condition that
> can result in lockups waiting for completions.  Adding the thread
> to (and removing the thread from) the wait queue for the completion
> is done outside the do loop in that function.  However, if the thread
> is woken up, the swake_up_locked() function will delete the entry
> from the wait queue.  If that happens and another thread sneaks
> in and decrements the done count in the completion to zero, the
> loop will go around again, but the thread will no longer be in the
> wait queue, so there is no way to wake it up.
> 
> Visually, here's a diagram from Sebastian Andrzej Siewior:
>   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.
> 
> Fixes: a04ff6b4ec4ee7e ("completion: Use simple wait queues")
> Signed-off-by: Corey Minyard <cminyard@xxxxxxxxxx>

Thanks!

Acked-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>

-- Steve

> ---
> Changes since v1:
> * Only move __prepare_to_swait() into the loop.  __prepare_to_swait()
>   handles if called when already in the list, and of course
>   __finish_swait() handles if the item is not queued on the
>   list.
> 
>  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);