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

[Aaron Tomlin]

On Wed, Sep 17, 2014 at 11:42:09PM +0200, Oleg Nesterov wrote:
> > 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
> 
> Aha. Perhaps you hit this bug under the older kernel?

I did. Sorry for the noise.

> "same workqueue" doesn't mean "same worker thread" today, every CPU can
> run up to ->max_active works. And for system_wq uses max_active = 256.
> 
> >   - 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
> 
> OK,
> 
> >   - 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 don't this this can happen, another worker threaf from worker_pool can
> handle this work.

Understood.

-- 
Aaron Tomlin
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