linux-kernel - Re: [PATCH] jffs2: Re-enable write-buffering after filesystem sync

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Date:	Fri, 1 Aug 2014 13:15:45 -0400
From:	Jeff Harris <jefftharris@...il.com>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	David Woodhouse <dwmw2@...radead.org>,
	linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Jeff Harris <jharris@...tell.com>
Subject: Re: [PATCH] jffs2: Re-enable write-buffering after filesystem sync

On Fri, Aug 1, 2014 at 12:56 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> On Fri, Aug 01, 2014 at 12:06:12PM -0400, Jeff Harris wrote:
>
> > +     spin_lock(&c->wbuf_dwork_lock);
> >       cancel_delayed_work_sync(&c->wbuf_dwork);
>
> Umm...  Usually ..._sync in function name is a sign of potential sleeper,
> and calling those under a spinlock is a bad idea.
>
> And looking at the definition of cancel_delayed_work_sync() turns up the
> following call chain: cancel_delayed_work_sync() -> __cancel_work_timer() ->
> flush_work() -> wait_for_completion(), which definitely isn't something
> you should ever do under a spinlock.

The jffs2_dirty_trigger function calls queue_delayed_work under the spinlock.
I suppose the flag could be reset after the cancel so the dirty
trigger would see
the false value.

>
>
> While we are at it, you follow that with
> > +     c->wbuf_queued = 0;
> > +     spin_lock(&c->wbuf_dwork_lock);
> which would be broken even if cancel_delayed_work_sync() hadn't blocked.
> That's easily fixed, of course, (s/lock/unlock/).  cancel_delayed_work_sync()
> under a spinlock is more serious...

Whoops, sorry about that.

Jeff
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