linux-kernel - Re: [PATCH] Fix queueing work if !bdi_cap_writeback

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Message-ID: <87sjan3c27.fsf@devron.myhome.or.jp>
Date:	Wed, 12 Sep 2012 17:00:48 +0900
From:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:	Fengguang Wu <fengguang.wu@...el.com>
Cc:	viro@...iv.linux.org.uk, jack@...e.cz, hch@....de,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix queueing work if !bdi_cap_writeback_dirty()

Fengguang Wu <fengguang.wu@...el.com> writes:

> On Wed, Sep 12, 2012 at 03:28:42AM +0900, OGAWA Hirofumi wrote:
>> 
>> If bdi has BDI_CAP_NO_WRITEBACK, bdi_forker_thread() doesn't start
>> writeback thread. This means there is no consumer of work item made
>> by bdi_queue_work().
>> 
>> This adds to checking of !bdi_cap_writeback_dirty(sb->s_bdi) before
>> calling bdi_queue_work(), otherwise queued work never be consumed.
>
> Thanks for catching this! Does this bug have any side effects other
> than memory leaking?
>
> It may be possible for some caller that actually expect it to do some
> work to make progress, otherwise will eventually block.  If so, we'll
> need to fix the caller.

If used custom bdi with BDI_CAP_NO_WRITEBACK, wait_for_completion()
(e.g. sync_inodes_sb()) will be blocked forever.

I tested by custom bdi with BDI_CAP_NO_WRITEBACK - sync(2) blocked
forever by this reason.

Thanks.
-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
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