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: <ZJl4C7aVk3gLLyMs@dhcp22.suse.cz>
Date:   Mon, 26 Jun 2023 13:35:39 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        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>,
        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 Mon 26-06-23 20:26:02, Tetsuo Handa wrote:
> On 2023/06/26 19:48, Peter Zijlstra wrote:
> > On Mon, Jun 26, 2023 at 06:25:56PM +0900, Tetsuo Handa wrote:
> >> 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.
> > 
> > Don't be like that... just hate on prink like the rest of us. In fact,
> > i've been patching out the actual printk code for years because its
> > unusable garbage.
> > 
> > Will this actually still be a problem once all the fancy printk stuff
> > lands? That shouldn't do synchronous prints except to 'atomic' consoles
> > by default IIRC.
> 
> Commit 1007843a9190 ("mm/page_alloc: fix potential deadlock on zonelist_update_seq
> seqlock") was applied to 4.14-stable trees, and CONFIG_PREEMPT_RT is available
> since 5.3. Thus, we want a fix which can be applied to 5.4-stable and later.
> This means that we can't count on all the fancy printk stuff being available.

Is there any reason to backport RT specific fixup to stable trees? I
mean seriously, is there any actual memory hotplug user using
PREEMPT_RT? I would be more than curious to hear the usecase.

-- 
Michal Hocko
SUSE Labs