linux-kernel - Re: [RFC PATCH] fs: Use a seperate wq for do_sync

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Message-ID: <20140917182202.GE19308@redhat.com>
Date:	Wed, 17 Sep 2014 20:22:02 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Aaron Tomlin <atomlin@...hat.com>
Cc:	linux-fsdevel@...r.kernel.org, viro@...iv.linux.org.uk,
	david@...morbit.com, bmr@...hat.com, jcastillo@...hat.com,
	mguzik@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] fs: Use a seperate wq for do_sync_work() to avoid
	a potential deadlock

On 09/17, Aaron Tomlin wrote:
>
> Since do_sync_work() is a deferred function it can block indefinitely by
> design. At present do_sync_work() is added to the global system_wq.
> As such a deadlock is theoretically possible between sys_unmount() and
> sync_filesystems():
>
>   * The current work fn on the system_wq (do_sync_work()) is blocked
>     waiting to aquire a sb's s_umount for reading.
>
>   * The "umount" task is the current owner of the s_umount in
>     question but is waiting for do_sync_work() to continue.
>     Thus we hit a deadlock situation.
>
I can't comment the patches in this area, but I am just curious...

Could you explain this deadlock in more details? I simply can't understand
what "waiting for do_sync_work()" actually means.

> This patch introduces a separate workqueue for do_sync_work() to avoid a
> the described deadlock.

The subject and the changelog do not match the patch, it doesn't add/use
another workqueue.

Oleg.

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