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

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Message-ID: <ff68da6a-8594-b218-c62b-4ae8e1ffae0a@ispras.ru>
Date: Thu, 8 Jun 2023 12:01:15 +0300
From: Vlad Efanov <vefanov@...ras.ru>
To: "Keller, Jacob E" <jacob.e.keller@...el.com>,
 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>,
 "stable@...nel.org" <stable@...nel.org>, Sven Eckelmann <sven@...fation.org>
Subject: Re: [PATCH 1/1] batman-adv: Broken sync while rescheduling delayed
 work

As far as I found the synchronization is provided by delayed work 
subsystem. It is based on the WORK_STRUCT_PENDING_BIT in work->data field.

The cancel_delayed_work_sync() atomically sets this bit and 
queue_delayed_work() checks it before scheduling new delayed work.


The problem is caused by the INIT_DELAYED_WORK() call inside 
batadv_dat_start_timer(). This call happens before the 
queue_delayed_work() call and clears this bit.


Best regards,

Vlad


On 08.06.2023 08:24, Keller, Jacob E wrote:
>
>> -----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