Re: [PATCH 06/11] super: Use acquire for SB_BORN check in super_cache_count()

[Jan Kara]

On Tue 25-08-26 17:54:17, Jinjie Ruan wrote:
> The active SB_BORN check in super_cache_count() pairs with the
> smp_store_release() in super_wake() when publishing a newly initialized
> superblock.
> 
> Replace the historical independent smp_rmb() barrier with an acquire load
> on sb->s_flags. This expresses the publish-subscribe pattern more clearly
> and allows weakly-ordered architectures like arm64 to utilize efficient
> native instructions (e.g., LDAR) instead of a full one-way barrier.
> 
> Also update the outdated comment to correctly point to super_wake().
> 
> Assisted-by: Gemini:Gemini-3.1
> Assisted-by: DeepSeek:DeepSeek-V3
> Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>

Looks good. Just one nit below. With that addressed feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

> diff --git a/fs/super.c b/fs/super.c
> index 05e443173038..5860daea6d40 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -267,12 +267,11 @@ static unsigned long super_cache_count(struct shrinker *shrink,
>  	 * However, if we are currently mounting the superblock, the underlying
>  	 * filesystem might be in a state of partial construction and hence it
>  	 * is dangerous to access it.  super_trylock_shared() uses a SB_BORN check
> -	 * to avoid this situation, so do the same here. The memory barrier is
> -	 * matched with the one in mount_fs() as we don't hold locks here.
> +	 * to avoid this situation, so do the same here. The acquire is matched
> +	 * with the smp_store_release() in super_wake() as we don't hold locks here.
>  	 */
> -	if (!(sb->s_flags & SB_BORN))
> +	if (!(smp_load_acquire(&sb->s_flags) & SB_BORN))
>  		return 0;

We have super_flags() helper exactly for checks like this so it would be
good to use it here.

								Honza

> -	smp_rmb();
>  
>  	if (sb->s_op && sb->s_op->nr_cached_objects &&
>  	    super_fs_objects_eligible(sc))
> -- 
> 2.34.1
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR