Re: [PATCH] Fix queueing work if !bdi_cap_writeback_dirty()

From: OGAWA Hirofumi
Date: Wed Sep 12 2012 - 04:00:51 EST

Fengguang Wu <fengguang.wu@xxxxxxxxx> 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.

OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
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