Re: [PATCH v2] slab: Fix obj_ext is mistakenly considered NULL due to race condition

[Harry Yoo]

On Thu, Oct 23, 2025 at 10:33:13PM +0800, Hao Ge wrote:
> From: Hao Ge <gehao@xxxxxxxxxx>
> 
> If two competing threads enter alloc_slab_obj_exts(), if the process
> that allocates the vector wins cmpxchg(), and the other thread mistakenly
> assume slab->obj_ext is still empty due to its own allocation failure.

Massaging this a little bit:

  "If two competing threads enter alloc_slab_obj_exts(), and the one that
   allocates the vector wins the cmpxchg(), the other thread that failed
   allocation mistakenly assumes that slab->obj_exts is still empty due to
   its own allocation failure."

> This
> will then trigger warnings enforced by CONFIG_MEM_ALLOC_PROFILING_DEBUG
> checks in the subsequent free path.
> 
> Therefore, let's add an additional check when the process that allocates
> the vector loses the cmpxchg()

You mean "when the process that failed to allocate the vector loses the
cmpxchg()"?

> Suggested-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
> Signed-off-by: Hao Ge <gehao@xxxxxxxxxx>
> ---
> v2: Revise the solution according to Harry's suggestion.
>     Add Suggested-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
> ---
>
>  mm/slub.c | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index d4403341c9df..d7bfec6c0171 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2052,9 +2052,9 @@ static inline void mark_objexts_empty(struct slabobj_ext *obj_exts)
>  	}
>  }
>  
> -static inline void mark_failed_objexts_alloc(struct slab *slab)
> +static inline bool mark_failed_objexts_alloc(struct slab *slab)
>  {
> -	cmpxchg(&slab->obj_exts, 0, OBJEXTS_ALLOC_FAIL);
> +	return cmpxchg(&slab->obj_exts, 0, OBJEXTS_ALLOC_FAIL) == 0;
>  }
>  
>  static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
> @@ -2076,7 +2076,7 @@ static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
>  #else /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */
>  
>  static inline void mark_objexts_empty(struct slabobj_ext *obj_exts) {}
> -static inline void mark_failed_objexts_alloc(struct slab *slab) {}
> +static inline bool mark_failed_objexts_alloc(struct slab *slab) { return false; }
>  static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
>  			struct slabobj_ext *vec, unsigned int objects) {}
>  
> @@ -2124,8 +2124,14 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>  				   slab_nid(slab));
>  	}
>  	if (!vec) {
> -		/* Mark vectors which failed to allocate */
> -		mark_failed_objexts_alloc(slab);
> +		/*
> +		 * Try to mark vectors which failed to allocate

nit:
                                                               ^ missing
							       period
							       (.) here

With the comments resolved,
Reviewed-by: Harry Yoo <harry.yoo@xxxxxxxxxx>

> +		 * If this operation fails, there may be a racing process
> +		 * that has already completed the allocation.
> +		 */
> +		if (!mark_failed_objexts_alloc(slab) &&
> +		    slab_obj_exts(slab))
> +			return 0;
>  
>  		return -ENOMEM;
>  	}
> -- 
> 2.25.1

-- 
Cheers,
Harry / Hyeonggon