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

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Message-ID: <0a0c768c-227d-c0cd-1b91-5a884d161c1b@I-love.SAKURA.ne.jp>
Date:   Mon, 26 Jun 2023 18:25:56 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
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>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Waiman Long <longman@...hat.com>, 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 2023/06/26 17:12, Sebastian Andrzej Siewior wrote:
> On 2023-06-24 15:54:12 [+0900], Tetsuo Handa wrote:
>> Why not to do the same on the end side?
>>
>>  static inline void do_write_seqcount_end(seqcount_t *s)
>>  {
>> - 	seqcount_release(&s->dep_map, _RET_IP_);
>>  	do_raw_write_seqcount_end(s);
>> +	seqcount_release(&s->dep_map, _RET_IP_);
>>  }
> 
> I don't have a compelling argument for doing it. It is probably better
> to release the lock from lockdep's point of view and then really release
> it (so it can't be acquired before it is released).

We must do it because this is a source of possible printk() deadlock.
Otherwise, I will nack on PATCH 2/2.

> 
> Looking at other locking primitives (spin_lock_unlock(),
> mutex_unlock(),…) that is what they do in the unlock path: lockdep
> annotation followed by the actual operation. Therefore I would keep the
> current ordering to remain in-sync with the other primitives.