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

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Jens Axboe <axboe@fb.com>, Ingo Molnar <mingo@kernel.org>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Chris Mason <clm@fb.com>, Dave Chinner <david@fromorbit.com>,
	Jan Kara <jack@suse.cz>, Josef Bacik <jbacik@fb.com>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Neil Brown <neilb@suse.de>, Christoph Hellwig <hch@lst.de>,
	Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug()
Date: Fri, 18 Sep 2015 09:12:38 -0700	[thread overview]
Message-ID: <CA+55aFyUPuAHrwHewH7356y=tAw=M+o_d0J5CqjjaDMzxN2AEA@mail.gmail.com> (raw)
In-Reply-To: <20150918155956.GZ3816@twins.programming.kicks-ass.net>

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