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

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Date:   Tue, 30 Jun 2020 12:53:55 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     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>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: wait_on_page_bit_common(TASK_KILLABLE, EXCLUSIVE) can miss
 wakeup?

On 06/30, Nicholas Piggin wrote:
>
> My patch is what actually introduced this ugly
> bit test, but do we even need it at all? If we do then it's
> under-commented, I can't see it wouldn't be racy though. Can we just
> get rid of it entirely?

But then we will need to move io_schedule() down, after test_and_set_bit().
And we will have the same problem with task->state != RUNNING. Plus more
complications with "behavior == DROP".

Oleg.