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Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug()

From Linus Torvalds <torvalds@linux-foundation.org>
Newsgroups linux.kernel
Subject Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug()
Date 2015-09-18 18:20 +0200
Message-ID <qa6ye-7HP-33@gated-at.bofh.it> (permalink)
References (6 earlier) <q9X1T-2y9-3@gated-at.bofh.it> <qa3K2-3Fh-21@gated-at.bofh.it> <qa4PM-5bi-33@gated-at.bofh.it> <qa5Vx-6Jf-59@gated-at.bofh.it> <qa6oy-7wu-29@gated-at.bofh.it>
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On Fri, Sep 18, 2015 at 8:59 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>
> See PREEMPT_ACTIVE being a recursion flag, we set it there so we won't
> preempt while we're already scheduling.

PREEMPT_ACTIVE does more than that. It really is a sign that "this is
not synchronous". It causes the scheduler to ignore the current task
flags (because it might already be TASK_SLEEPING, but we aren't
_actually_ ready to sleep yet) etc.

So no. It's not "you can't be preempted during scheduling". That's the
*normal* preempt count, and all scheduling calls end up setting that
some way (ie "schedule()" just does preempt_disable()).

So I disagree with your notion that it's a recursion flag. It is
absolutely nothing of the sort. It gets set by preemption - and,
somewhat illogically, by cond_resched().

The fact that cond_resched() sets it is *probably* because some of the
callers end up calling it from page fault paths etc, and the same
"ignore TASK_SLEEPING etc" rules apply. But it does mean that
"cond_resched()" is a bit misleaning as a name. It's really a
"cond_preempt()".

                 Linus
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Thread

[PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Chris Mason <clm@fb.com> - 2015-09-11 21:40 +0200
  Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-11 22:10 +0200
    Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-11 22:40 +0200
      Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Josef Bacik <jbacik@fb.com> - 2015-09-11 22:50 +0200
        Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-11 23:10 +0200
          Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-12 00:10 +0200
            Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Chris Mason <clm@fb.com> - 2015-09-12 01:20 +0200
              Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-12 01:40 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-12 03:00 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Chris Mason <clm@fb.com> - 2015-09-12 04:20 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-12 04:30 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Chris Mason <clm@fb.com> - 2015-09-13 01:10 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-13 01:30 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Chris Mason <clm@fb.com> - 2015-09-13 01:50 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Chris Mason <clm@fb.com> - 2015-09-13 15:20 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Dave Chinner <david@fromorbit.com> - 2015-09-14 01:00 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Dave Chinner <david@fromorbit.com> - 2015-09-14 01:20 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-14 22:10 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Jan Kara <jack@suse.cz> - 2015-09-16 22:00 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Chris Mason <clm@fb.com> - 2015-09-16 22:10 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Dave Chinner <david@fromorbit.com> - 2015-09-17 00:20 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Dave Chinner <david@fromorbit.com> - 2015-09-17 02:40 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-17 03:20 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Dave Chinner <david@fromorbit.com> - 2015-09-17 04:20 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-17 21:40 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Chris Mason <clm@fb.com> - 2015-09-18 00:50 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-18 01:10 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Chris Mason <clm@fb.com> - 2015-09-18 02:00 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Dave Chinner <david@fromorbit.com> - 2015-09-18 02:40 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-18 04:00 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Dave Chinner <david@fromorbit.com> - 2015-09-18 07:50 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-18 08:10 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-18 08:10 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Jens Axboe <axboe@fb.com> - 2015-09-18 16:30 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Chris Mason <clm@fb.com> - 2015-09-18 15:20 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Jens Axboe <axboe@fb.com> - 2015-09-18 16:30 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-18 17:40 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Peter Zijlstra <peterz@infradead.org> - 2015-09-18 18:10 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Peter Zijlstra <peterz@infradead.org> - 2015-09-18 18:10 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-18 18:20 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Dave Chinner <david@fromorbit.com> - 2015-09-19 01:20 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Jan Kara <jack@suse.cz> - 2015-09-21 11:30 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during  blk_finish_plug() Andrew Morton <akpm@linux-foundation.org> - 2015-09-21 22:30 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Dave Chinner <david@fromorbit.com> - 2015-09-18 01:10 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-18 01:20 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Chris Mason <clm@fb.com> - 2015-09-17 05:50 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Dave Chinner <david@fromorbit.com> - 2015-09-17 06:40 +0200
                Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Chris Mason <clm@fb.com> - 2015-09-17 14:20 +0200
          Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Chris Mason <clm@fb.com> - 2015-09-12 01:10 +0200
            Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-12 01:20 +0200

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