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

[Aaron Tomlin <atomlin@redhat.com>]

On Wed, Sep 17, 2014 at 08:22:02PM +0200, Oleg Nesterov wrote:
> 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.

Hopefully this helps:

	           "umount"                                      "events/1"

sys_umount					    sysrq_handle_sync
  deactivate_super(sb)				      emergency_sync
  {						    	schedule_work(work)
    ...						    	  queue_work(system_wq, work)
    down_write(&s->s_umount)			    	    do_sync_work(work)
    ...						      	      sync_filesystems(0)
    kill_block_super(s)				    		...
      generic_shutdown_super(sb)		    		down_read(&sb->s_umount)
      // sop->put_super(sb)
      ext4_put_super(sb)
	invalidate_bdev(sb->s_bdev)
	  lru_add_drain_all()
	    for_each_online_cpu(cpu) {
	      schedule_work_on(cpu, work)
		queue_work_on(cpu, system_wq, work)
		...
	    }
  }

  - Both lru_add_drain and do_sync_work work items are added to
    the same global system_wq

  - The current work fn on the system_wq is do_sync_work and is
    blocked waiting to aquire an 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.


-- 
Aaron Tomlin