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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun, 3 Nov 2019 08:45:25 -0800
Message-ID: <CAHk-=wgE-veRb7+mw9oMmsD97BLnL+q8Gxu0QRrK65S2yQfMdQ@mail.gmail.com>
Subject: Re: [PATCH v3 1/1] media: vivid: Fix wrong locking that causes race
 conditions on streaming stop
To:     Alexander Popov <alex.popov@linux.com>
Cc:     Hans Verkuil <hverkuil-cisco@xs4all.nl>,
        Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
        Security Officers <security@kernel.org>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Sat, Nov 2, 2019 at 12:03 PM Alexander Popov <alex.popov@linux.com> wrote:
>
> -               mutex_lock(&dev->mutex);
> +               if (!mutex_trylock(&dev->mutex)) {
> +                       schedule_timeout(1);
> +                       continue;
> +               }
> +

I just realized that this too is wrong. It _works_, but because it
doesn't actually set the task state to anything particular before
scheduling, it's basically pointless. It calls the scheduler, but it
won't delay anything, because the task stays runnable.

So what you presumably want to use is either "cond_resched()" (to make
sure others get to run with no delay) or
"schedule_timeout_uninterruptible(1)" which actually sets the process
state to TASK_UNINTERRUPTIBLE.

The above works, but it's basically nonsensical. My bad for not
noticing earlier.

               Linus