linux-kernel - Re: [PATCH v2 1/2] seqlock: Do the lockdep annotation before locking in do_write_seqcount_begin

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Date:   Mon, 26 Jun 2023 13:56:18 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        John Ogness <john.ogness@...utronix.de>,
        Michal Hocko <mhocko@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Thomas Gleixner <tglx@...utronix.de>,
        Waiman Long <longman@...hat.co>, Will Deacon <will@...nel.org>
Subject: Re: [PATCH v2 1/2] seqlock: Do the lockdep annotation before locking
 in do_write_seqcount_begin_nested()

On Fri, Jun 23, 2023 at 07:12:31PM +0200, Sebastian Andrzej Siewior wrote:
> It was brought up by Tetsuo that the following sequence
>    write_seqlock_irqsave()
>    printk_deferred_enter()
> 
> could lead to a deadlock if the lockdep annotation within
> write_seqlock_irqsave() triggers. The problem is that the sequence
> counter is incremented before the lockdep annotation is performed. The
> lockdep splat would then attempt to invoke printk() but the reader side,
> of the same seqcount, could have a tty_port::lock acquired waiting for
> the sequence number to become even again.
> 
> The other lockdep annotations come before the actual locking because "we
> want to see the locking error before it happens". There is no reason why
> seqcount should be different here.
> 
> Do the lockdep annotation first then perform the locking operation (the
> sequence increment).
> 
> Fixes: 1ca7d67cf5d5a ("seqcount: Add lockdep functionality to seqcount/seqlock structures")
> Reported-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> Link: https://lore.kernel.org/20230621130641.-5iueY1I@linutronix.de
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

Acked-by: Mel Gorman <mgorman@...hsingularity.net>

-- 
Mel Gorman
SUSE Labs