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

Mel Gorman
2023-06-26T13:14:46Z

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