linux-kernel - Re: [PATCH v2] igb: Fix watchdog

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Message-ID: <aEgokTyzDrZ6p4aL@21d8f0102f10>
Date: Tue, 10 Jun 2025 15:44:01 +0300
From: Ian Ray <ian.ray@...ealthcare.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: horms@...nel.org, Tony Nguyen <anthony.l.nguyen@...el.com>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	brian.ruley@...ealthcare.com, intel-wired-lan@...ts.osuosl.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] igb: Fix watchdog_task race with shutdown

On Mon, Jun 09, 2025 at 04:10:39PM -0700, Jakub Kicinski wrote:
> On Mon, 9 Jun 2025 09:32:58 +0300 Ian Ray wrote:
> > On Thu, Jun 05, 2025 at 06:43:39PM -0700, Jakub Kicinski wrote:
> > > On Tue,  3 Jun 2025 11:09:49 +0300 Ian Ray wrote:
> > > >       set_bit(__IGB_DOWN, &adapter->state);
> > > > +     timer_delete_sync(&adapter->watchdog_timer);
> > > > +     timer_delete_sync(&adapter->phy_info_timer);
> > > > +
> > > > +     cancel_work_sync(&adapter->watchdog_task);
> > >
> > > This doesn't look very race-proof as watchdog_task
> > > can schedule the timer as its last operation?
> >
> > Thanks for the reply.  __IGB_DOWN is the key to this design.
> >
> > If watchdog_task runs *before* __IGB_DOWN is set, then the
> > timer is stopped (by this patch) as required.
> >
> > However, if watchdog_task runs *after* __IGB_DOWN is set,
> > then the timer will not even be started (by watchdog_task).
> 
> Well, yes, but what if the two functions run *simultaneously*
> There is no mutual exclusion between these two pieces of code AFAICT

Thank you for clarifying.

IIUC set_bit() is an atomic operation (via bitops.h), and so
my previous comment still stands.

(Sorry if I have misunderstood your question.)

Either watchdog_task runs just before __IGB_DOWN is set (and
the timer is stopped by this patch) -- or watchdog_task runs
just after __IGB_DOWN is set (and thus the timer will not be
restarted).

In both cases, the final cancel_work_sync ensures that the
watchdog_task completes before igb_down() continues.

Regards,
Ian