netdev - Re: [PATCH net] ptp: prevent possible ABBA deadlock in ptp_clock

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Message-ID: <20250707171118.55fc88cc@kernel.org>
Date: Mon, 7 Jul 2025 17:11:18 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Jeongjun Park <aha310510@...il.com>
Cc: richardcochran@...il.com, andrew+netdev@...n.ch, davem@...emloft.net,
 edumazet@...gle.com, pabeni@...hat.com, yangbo.lu@....com,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 syzbot+7cfb66a237c4a5fb22ad@...kaller.appspotmail.com
Subject: Re: [PATCH net] ptp: prevent possible ABBA deadlock in
 ptp_clock_freerun()

On Sat,  5 Jul 2025 23:50:31 +0900 Jeongjun Park wrote:
> ABBA deadlock occurs in the following scenario:
> 
>        CPU0                           CPU1
>        ----                           ----
>   n_vclocks_store()
>     lock(&ptp->n_vclocks_mux) [1]
>                                      pc_clock_adjtime()
>                                        lock(&clk->rwsem) [2]
>                                        ...
>                                        ptp_clock_freerun()
>                                          ptp_vclock_in_use()
>                                            lock(&ptp->n_vclocks_mux) [3]
>     ptp_clock_unregister()
>       posix_clock_unregister()
>         lock(&clk->rwsem) [4]
> 
> To solve this with minimal patches, we should change ptp_clock_freerun()
> to briefly release the read lock before calling ptp_vclock_in_use() and
> then re-lock it when we're done.

Dropping locks randomly is very rarely the correct fix.

Either way - you forgot to CC Vladimir, again.
-- 
pw-bot: cr