linux-kernel - Re: [PATCH v3 2/2] mm/page_alloc: Use write_seqlock_irqsave() instead write_seqlock() + local_irq

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20230626131438.54bupbuujbqe4b5a@techsingularity.net>
Date:   Mon, 26 Jun 2023 14:14:38 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Michal Hocko <mhocko@...e.com>, 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>,
        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.com>, Will Deacon <will@...nel.org>
Subject: Re: [PATCH v3 2/2] mm/page_alloc: Use write_seqlock_irqsave()
 instead write_seqlock() + local_irq_save().

On Fri, Jun 23, 2023 at 10:15:17PM +0200, Sebastian Andrzej Siewior wrote:
> __build_all_zonelists() acquires zonelist_update_seq by first disabling
> interrupts via local_irq_save() and then acquiring the seqlock with
> write_seqlock(). This is troublesome and leads to problems on
> PREEMPT_RT. The problem is that the inner spinlock_t becomes a sleeping
> lock on PREEMPT_RT and must not be acquired with disabled interrupts.
> 
> The API provides write_seqlock_irqsave() which does the right thing in
> one step.
> printk_deferred_enter() has to be invoked in non-migrate-able context to
> ensure that deferred printing is enabled and disabled on the same CPU.
> This is the case after zonelist_update_seq has been acquired.
> 
> There was discussion on the first submission that the order should be:
> 	local_irq_disable();
> 	printk_deferred_enter();
> 	write_seqlock();
> 
> to avoid pitfalls like having an unaccounted printk() coming from
> write_seqlock_irqsave() before printk_deferred_enter() is invoked. The
> only origin of such a printk() can be a lockdep splat because the
> lockdep annotation happens after the sequence count is incremented.
> This is exceptional and subject to change.
> 
> It was also pointed that PREEMPT_RT can be affected by the printk
> problem since its write_seqlock_irqsave() does not really disable
> interrupts. This isn't the case because PREEMPT_RT's printk
> implementation differs from the mainline implementation in two important
> aspects:
> - Printing happens in a dedicated threads and not at during the
>   invocation of printk().
> - In emergency cases where synchronous printing is used, a different
>   driver is used which does not use tty_port::lock.
> 
> Acquire zonelist_update_seq with write_seqlock_irqsave() and then defer
> printk output.
> 
> Fixes: 1007843a91909 ("mm/page_alloc: fix potential deadlock on zonelist_update_seq seqlock")
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Acked-by: Michal Hocko <mhocko@...e.com>

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

-- 
Mel Gorman
SUSE Labs