From: Oliver Neukum <oneukum@suse.com>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: linux-usb@vger.kernel.org
Subject: Re: [RFC] deadlock with flush_work() in UAS
Date: Tue, 18 Jun 2019 17:29:34 +0200 [thread overview]
Message-ID: <1560871774.3184.16.camel@suse.com> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1906181120550.1659-100000@iolanthe.rowland.org>
Am Dienstag, den 18.06.2019, 11:29 -0400 schrieb Alan Stern:
> On Tue, 18 Jun 2019, Oliver Neukum wrote:
>
> > Hi,
> >
> > looking at those deadlocks it looks to me like UAS can
> > deadlock on itself. What do you think?
> >
> > Regards
> > Oliver
> >
> > From 2d497f662e6c03fe9e0a75e05b64d52514e890b3 Mon Sep 17 00:00:00 2001
> > From: Oliver Neukum <oneukum@suse.com>
> > Date: Tue, 18 Jun 2019 15:03:56 +0200
> > Subject: [PATCH] UAS: fix deadlock in error handling and PM flushing work
> >
> > A SCSI error handler and block runtime PM must not allocate
> > memory with GFP_KERNEL. Furthermore they must not wait for
> > tasks allocating memory with GFP_KERNEL.
> > That means that they cannot share a workqueue with arbitrary tasks.
> >
> > Fix this for UAS using a private workqueue.
>
> I'm not so sure that one long-running task in a workqueue will block
> other tasks. Workqueues have variable numbers of threads, added and
> removed on demand. (On the other hand, when new threads need to be
> added the workqueue manager probably uses GFP_KERNEL.)
Do we have a guarantee it will reschedule already scheduled works?
The deadlock would be something like
uas_pre_reset() -> uas_wait_for_pending_cmnds() ->
flush_work(&devinfo->work) -> kmalloc() -> DEADLOCK
You can also make this chain with uas_suspend()
> Even if you disagree, perhaps we should have a global workqueue with a
> permanently set noio flag. It could be shared among multiple drivers
> such as uas and the hub driver for purposes like this. (In fact, the
> hub driver already has its own dedicated workqueue.)
That is a good idea. But does UAS need WQ_MEM_RECLAIM?
Regards
Oliver
next prev parent reply other threads:[~2019-06-18 15:43 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-18 14:53 [RFC] deadlock with flush_work() in UAS Oliver Neukum
2019-06-18 15:29 ` Alan Stern
2019-06-18 15:29 ` Oliver Neukum [this message]
2019-06-18 15:59 ` Alan Stern
2019-06-20 14:10 ` Tejun Heo
2019-06-24 8:56 ` Oliver Neukum
2019-06-24 14:22 ` Alan Stern
2019-06-26 8:10 ` Oliver Neukum
2019-06-26 14:38 ` Alan Stern
2019-07-01 11:02 ` Oliver Neukum
2019-07-01 14:20 ` Alan Stern
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