Re: [RFC PATCH] fs: Use a seperate wq for do_sync_work() to avoid a potential deadlock

[Oleg Nesterov <oleg@redhat.com>]

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.