linux-kernel - Re: livelock in __writeback_inodes

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Date:	Tue, 11 Dec 2012 20:26:22 -0600
From:	Simon Jeons <simon.jeons@...il.com>
To:	Dave Jones <davej@...hat.com>
Cc:	Fengguang Wu <fengguang.wu@...el.com>, linux-mm@...ck.org,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org
Subject: Re: livelock in __writeback_inodes_wb ?

On Tue, 2012-12-11 at 09:29 -0500, Dave Jones wrote:
> On Tue, Dec 11, 2012 at 04:23:27PM +0800, Fengguang Wu wrote:
>  > On Wed, Nov 28, 2012 at 09:55:15AM -0500, Dave Jones wrote:
>  > > We had a user report the soft lockup detector kicked after 22
>  > > seconds of no progress, with this trace..
>  > 
>  > Where is the original report? The reporter may help provide some clues
>  > on the workload that triggered the bug.
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=880949 
> 
>  > The bug reporter should know best whether there are heavy IO.
>  > 
>  > However I suspect it's not directly caused by heavy IO: we will
>  > release &wb->list_lock before each __writeback_single_inode() call,
>  > which starts writeback IO for each inode.
>  > 
>  > > Should there be something in this loop periodically poking
>  > > the watchdog perhaps ?
>  > 
>  > It seems we failed to release &wb->list_lock in wb_writeback() for
>  > long time (dozens of seconds). That is, the inode_sleep_on_writeback()
>  > is somehow not called. However it's not obvious to me how come this
>  > can happen..
> 
> Right, it seems that we only drop the lock when there is more work to do.
> And if there is no work to do, then we would have bailed from the loop.

If no work to do, lock will be dropped after for loop.

> 
> mysterious.
> 
> 	Dave
> 
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