Re: [PATCH v3 2/2] mm/page_alloc: Use write_seqlock_irqsave() instead write_seqlock() + local_irq_save().

From: Mel Gorman <mgorman@techsingularity.net>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Michal Hocko <mhocko@suse.com>,
	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>,
	Peter Zijlstra <peterz@infradead.org>,
	Petr Mladek <pmladek@suse.com>,
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
	Thomas Gleixner <tglx@linutronix.de>,
	Waiman Long <longman@redhat.com>, Will Deacon <will@kernel.org>
Subject: Re: [PATCH v3 2/2] mm/page_alloc: Use write_seqlock_irqsave() instead write_seqlock() + local_irq_save().
Date: Mon, 26 Jun 2023 14:14:38 +0100	[thread overview]
Message-ID: <20230626131438.54bupbuujbqe4b5a@techsingularity.net> (raw)
In-Reply-To: <20230623201517.yw286Knb@linutronix.de>

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@linutronix.de>
> Acked-by: Michal Hocko <mhocko@suse.com>

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

-- 
Mel Gorman
SUSE Labs