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

[David Hildenbrand <david@redhat.com>]

On 21.06.23 12:40, 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 because the inner spinlock_t is now 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.
> 
> 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>
> ---
>   mm/page_alloc.c | 11 ++++-------
>   1 file changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 47421bedc12b7..99b7e7d09c5c0 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5808,11 +5808,10 @@ 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
> @@ -5820,7 +5819,6 @@ static void __build_all_zonelists(void *data)
>   	 * 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@redhat.com>

-- 
Cheers,

David / dhildenb