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

[David Hildenbrand]

On 23.06.23 22:15, 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@xxxxxxxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> v2…v3
>    - Update comment as per Michal's suggestion.
>
> v1…v2:
>    - Improve commit description
>
>   mm/page_alloc.c | 15 ++++++---------
>   1 file changed, 6 insertions(+), 9 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 47421bedc12b7..440e9af67b48d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5808,19 +5808,17 @@ static void __build_all_zonelists(void *data)
>   	unsigned long flags;
>   
>   	/*
> -	 * Explicitly disable this CPU's interrupts before taking seqlock
> -	 * to prevent any IRQ handler from calling into the page allocator
> -	 * (e.g. GFP_ATOMIC) that could hit zonelist_iter_begin and livelock.
> +	 * The zonelist_update_seq must be acquired with irqsave because the
> +	 * reader can be invoked from IRQ with GFP_ATOMIC.
>   	 */
> -	local_irq_save(flags);
> +	write_seqlock_irqsave(&zonelist_update_seq, flags);
>   	/*
> -	 * Explicitly disable this CPU's synchronous printk() before taking
> -	 * seqlock to prevent any printk() from trying to hold port->lock, for
> +	 * Also disable synchronous printk() to prevent any printk() from
> +	 * trying to hold port->lock, for
>   	 * tty_insert_flip_string_and_push_buffer() on other CPU might be
>   	 * calling kmalloc(GFP_ATOMIC | __GFP_NOWARN) with port->lock held.
>   	 */
>   	printk_deferred_enter();
> -	write_seqlock(&zonelist_update_seq);
>   
>   #ifdef CONFIG_NUMA
>   	memset(node_load, 0, sizeof(node_load));
> @@ -5857,9 +5855,8 @@ static void __build_all_zonelists(void *data)
>   #endif
>   	}
>   
> -	write_sequnlock(&zonelist_update_seq);
>   	printk_deferred_exit();
> -	local_irq_restore(flags);
> +	write_sequnlock_irqrestore(&zonelist_update_seq, flags);
>   }
>   
>   static noinline void __init

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers,

David / dhildenb