linux-kernel - Re: wait_on_page_bit_common(TASK_KILLABLE, EXCLUSIVE) can miss wakeup?

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Date:   Mon, 29 Jun 2020 16:02:46 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Davidlohr Bueso <dave@...olabs.net>, Jan Kara <jack@...e.cz>,
        Lukas Czerner <lczerner@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: wait_on_page_bit_common(TASK_KILLABLE, EXCLUSIVE) can miss
 wakeup?

On 06/29, Nicholas Piggin wrote:
>
> prepare_to_wait_event() has a pretty good pattern (and comment), I would
> favour using that (test the signal when inserting on the waitqueue).
>
> @@ -1133,6 +1133,15 @@ static inline int wait_on_page_bit_common(wait_queue_head_t *q,
>         for (;;) {
>                 spin_lock_irq(&q->lock);
>
> +               if (signal_pending_state(state, current)) {
> +                       /* Must not lose an exclusive wake up, see
> +                        * prepare_to_wait_event comment */
> +                       list_del_init(&wait->entry);
> +                       spin_unlock_irq(&q->lock);
> +                       ret = -EINTR;

Basically this is what my patch in the 1st email does. But note that we can't
just set "ret = -EINTR" here, we will need to clear "ret" if test_and_set_bit()
below succeeds. That is why I used another "int intr" variable.

Oleg.