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

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Message-ID: <20140801165607.GM18016@ZenIV.linux.org.uk>
Date:	Fri, 1 Aug 2014 17:56:07 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Jeff Harris <jefftharris@...il.com>
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 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.

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...
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