linux-kernel - Re: [PATCHv2] thunderbolt: do not double dequeue a request

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Message-ID: <jdupmjvntywimlzlhvq3rfsiwmlox6ssdtdncfe3mmo3wonzta@qwlb3wuosv66>
Date: Thu, 27 Mar 2025 23:37:35 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>, 
	Andreas Noever <andreas.noever@...il.com>, Michael Jamet <michael.jamet@...el.com>, 
	Mika Westerberg <westeri@...nel.org>, Yehezkel Bernat <YehezkelShB@...il.com>, 
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCHv2] thunderbolt: do not double dequeue a request

On (25/03/27 16:20), Mika Westerberg wrote:
> > On (25/03/27 15:37), Mika Westerberg wrote:
> > > > Another possibility can be tb_cfg_request_sync():
> > > > 
> > > > tb_cfg_request_sync()
> > > >  tb_cfg_request()
> > > >   schedule_work(&req->work) -> tb_cfg_request_dequeue()
> > > >  tb_cfg_request_cancel()
> > > >   schedule_work(&req->work) -> tb_cfg_request_dequeue()
> > > 
> > > Not sure about this one because &req->work will only be scheduled once the
> > > second schedule_work() should not queue it again (as far as I can tell).
> > 
> > If the second schedule_work() happens after a timeout, that's what
> > !wait_for_completion_timeout() does, then the first schedule_work()
> > can already execute the work by that time, and then we can schedule
> > the work again (but the request is already dequeued).  Am I missing
> > something?
> 
> schedule_work() does not schedule the work again if it is already
> scheduled.

Yes, if it's scheduled.  If it's already executed then we can schedule
again.

	tb_cfg_request_sync() {
	 tb_cfg_request()
	   schedule_work()
	                        executes tb_cfg_request_dequeue
	 wait_for_completion_timeout()
	   schedule_work()
	                        executes tb_cfg_request_dequeue again
	}

I guess there can be enough delay (for whatever reason, not only
wait_for_completion_timeout(), but maybe also preemption) between
two schedule_work calls?

> > The 0xdead000000000122 deference is a LIST_POISON on x86_64, which
> > is set explicitly in list_del(), so I'd say I'm fairly confident
> > that we have a double list_del() in tb_cfg_request_dequeue().
> 
> Yes, I agree but since I have not seen any similar reports (sans what I saw
> ages ago), I would like to be sure the issue you see is actually fixed with
> the patch (and that there are no unexpected side-effects). ;-)

Let me see what I can do (we don't normally apply patches that
were not in the corresponding subsystem tree).

In the meantime, do you have a subsystem/driver tree that is exposed
to linux-next?  If so, would be cool if you can pick up the patch so
that it can get some extra testing via linux-next.