linux-kernel - Re: [PATCH] batman-adv: Broken sync while rescheduling delayed work

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Date:   Fri, 26 May 2023 20:35:25 +0300
From:   Vlad Efanov <vefanov@...ras.ru>
To:     Sven Eckelmann <sven@...fation.org>,
        Marek Lindner <mareklindner@...mailbox.ch>
Cc:     Simon Wunderlich <sw@...onwunderlich.de>,
        Antonio Quartulli <a@...table.cc>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        b.a.t.m.a.n@...ts.open-mesh.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, lvc-project@...uxtesting.org
Subject: Re: [PATCH] batman-adv: Broken sync while rescheduling delayed work

Sven,


cancel_delayed_work_sync() and queue_delayed_work()

use WORK_STRUCT_PENDING_BIT in work->data to synchronize.

INIT_DELAYED_WORK() clears this bit.


The situation is :  __cancel_work_timer() sets WORK_STRUCT_PENDING_BIT

but INIT_DELAYED_WORK() in batadv_dat_start_timer() clears it

and queue_delayed_work() schedules new work.


Best regards,

Vlad.

On 26.05.2023 19:49, Sven Eckelmann wrote:
> On Friday, 26 May 2023 18:16:32 CEST Vladislav Efanov 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.
> You are most likely right but could you please point out what in the worker is
> checked by the workqueue code that prevents it from being scheduled again?
> (and which seems to be overwritten as your patch seems to suggest)
>
> I think __cancel_work_timer marked the work as canceling but
> batadv_dat_start_timer reinitialized the worked (thus removing this important
> state). Would be nice if you could either correct me or confirm what I think to
> remember.
>
> Kind regards,
> 	Sven