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Date:   Mon, 26 Jun 2023 15:13:02 +0200
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        "Luis Claudio R. Goncalves" <lgoncalv@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Ingo Molnar <mingo@redhat.com>,
        John Ogness <john.ogness@linutronix.de>,
        Mel Gorman <mgorman@techsingularity.net>,
        Michal Hocko <mhocko@suse.com>, Petr Mladek <pmladek@suse.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Waiman Long <longman@redhat.com>, Will Deacon <will@kernel.org>
Subject: Re: [PATCH v2 1/2] seqlock: Do the lockdep annotation before locking
 in do_write_seqcount_begin_nested()
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On 2023-06-26 12:48:31 [+0200], Peter Zijlstra wrote:
>=20
> 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.

The fancy printk stuff should have synchronous printing in emergency
situations and threaded printing by default. But then I hear John saying
that there might be push back because atomic consoles may not be
available everywhere=E2=80=A6

Anyway, the requirement for the deadlock to trigger, that Tetsuo Handa
is concerned here:
- lockdep enabled
- console and tty in use.
- tty_insert_flip_string_and_push_buffer() on one CPU with
  tty_port::lock acquired followed with a memory allocation blocking on
  read_seqbegin(&zonelist_update_seq) due the writer
- memory hotplug =3D> __build_all_zonelists() acquiring
  write_seqlock(&zonelist_update_seq) and now lockdep creates a splat.
  This is accounted for if the lockdep annotation comes first (1/2 of
  the series). But the unlock annotation of the seq unlock operation may
  still create a splat so a possibility for a deadlock remains.

The requirement is _high_ and it hardly justifies ugly code.

Sebastian