linux-kernel - Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish

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Date:	Fri, 18 Sep 2015 09:12:38 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Jens Axboe <axboe@...com>, Ingo Molnar <mingo@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Chris Mason <clm@...com>, Dave Chinner <david@...morbit.com>,
	Jan Kara <jack@...e.cz>, Josef Bacik <jbacik@...com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Neil Brown <neilb@...e.de>, Christoph Hellwig <hch@....de>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug()

On Fri, Sep 18, 2015 at 8:59 AM, Peter Zijlstra <peterz@...radead.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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