netdev - RE: [PATCH 1/1] batman-adv: Broken sync while rescheduling delayed work

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Date: Thu, 8 Jun 2023 05:24:12 +0000
From: "Keller, Jacob E" <jacob.e.keller@...el.com>
To: Jakub Kicinski <kuba@...nel.org>, Simon Wunderlich <sw@...onwunderlich.de>
CC: "davem@...emloft.net" <davem@...emloft.net>, "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>, "b.a.t.m.a.n@...ts.open-mesh.org"
	<b.a.t.m.a.n@...ts.open-mesh.org>, Vladislav Efanov <VEfanov@...ras.ru>,
	"stable@...nel.org" <stable@...nel.org>, Sven Eckelmann <sven@...fation.org>
Subject: RE: [PATCH 1/1] batman-adv: Broken sync while rescheduling delayed
 work



> -----Original Message-----
> From: Jakub Kicinski <kuba@...nel.org>
> Sent: Wednesday, June 7, 2023 10:01 PM
> To: Simon Wunderlich <sw@...onwunderlich.de>
> Cc: davem@...emloft.net; netdev@...r.kernel.org; b.a.t.m.a.n@...ts.open-
> mesh.org; Vladislav Efanov <VEfanov@...ras.ru>; stable@...nel.org; Sven
> Eckelmann <sven@...fation.org>
> Subject: Re: [PATCH 1/1] batman-adv: Broken sync while rescheduling delayed
> work
> 
> On Wed,  7 Jun 2023 17:55:15 +0200 Simon Wunderlich wrote:
> > The reason for these issues is the lack of synchronization. Delayed
> > work (batadv_dat_purge) schedules new timer/work while the device
> > is being deleted. As the result new timer/delayed work is set after
> > cancel_delayed_work_sync() was called. So after the device is freed
> > the timer list contains pointer to already freed memory.
> 
> I guess this is better than status quo but is the fix really complete?
> We're still not preventing the timer / work from getting scheduled
> and staying alive after the netdev has been freed, right?

Yea, I would expect some synchronization mechanism to ensure that after cancel_delayed_work_sync() you can't queue the work again.

I know for timers there is recently timer_shutdown_sync() which can be used to guarantee a timer can't re-arm at all, and its intended for some situations where there is a cyclic dependency...

Thanks,
Jake