Re: v5.10 and less needs a fix up for __build_all_zonelists()

On Fri, Nov 03, 2023 at 12:35:29PM +0100, Sebastian Andrzej Siewior wrote:
> Hi!
> 
> upstream landed commit
> 	a2ebb51575828 ("mm/page_alloc: use write_seqlock_irqsave() instead write_seqlock() + local_irq_save().")
> 
> a while ago after Luis pointed me at the actual issue. The v6.1-RT and
> v5.15-RT received the proper backport via -stable and are good.
> v5.10-RT has the commit
>     7bdd3bd5143a4 ("Revert "mm/page_alloc: fix potential deadlock on zonelist_update_seqseqlock"")
> 
> which I may have suggested at the time but requires now something
> proper. In reality it may not matter because nobody is using
> memory-hotplug but is should be fixed properly. Also it (as in v5.10)
> affects non-RT users. In less than v5.10 it affects possible RT users.
> 
> The fix for 5.10-RT is just the snippet which is the resolution if the
> initial commit would be backported to the v5.10 which did not happen.
> Everything less than v5.10 should apply commit a2ebb51575828 mentioned
> earlier.

Thank you for the heads-up, Sebastian!

I will finish the work on v5.10.199-rt97-rc1 and then release an RT-only
update with the fix below.

Luis

> Sebastian
> 
> -----8<---
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3760a06932a6e..f6b3a46ac87d5 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6043,8 +6043,9 @@ static void __build_all_zonelists(void *data)
>  	int nid;
>  	int __maybe_unused cpu;
>  	pg_data_t *self = data;
> +	unsigned long flags;
>  
> -	write_seqlock(&zonelist_update_seq);
> +	write_seqlock_irqsave(&zonelist_update_seq, flags);
>  
>  #ifdef CONFIG_NUMA
>  	memset(node_load, 0, sizeof(node_load));
> @@ -6077,7 +6078,7 @@ static void __build_all_zonelists(void *data)
>  #endif
>  	}
>  
> -	write_sequnlock(&zonelist_update_seq);
> +	write_sequnlock_irqrestore(&zonelist_update_seq, flags);
>  }
>  
>  static noinline void __init
> 
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