Re: [PATCH RFC v2 07/10] slab: determine barn status racily outside of lock

[Vlastimil Babka <vbabka@suse.cz>]

On 2/25/25 09:54, Harry Yoo wrote:
> On Fri, Feb 14, 2025 at 05:27:43PM +0100, Vlastimil Babka wrote:
>> The possibility of many barn operations is determined by the current
>> number of full or empty sheaves. Taking the barn->lock just to find out
>> that e.g. there are no empty sheaves results in unnecessary overhead and
>> lock contention. Thus perform these checks outside of the lock with a
>> data_race() annotated variable read and fail quickly without taking the
>> lock.
>> 
>> Checks for sheaf availability that racily succeed have to be obviously
>> repeated under the lock for correctness, but we can skip repeating
>> checks if there are too many sheaves on the given list as the limits
>> don't need to be strict.
>> 
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> 
> Looks good to me,
> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
> 
> in kmem_cache_return_sheaf:
>> if (!pcs->spare) {                                                      
>> 	pcs->spare = sheaf;                                             
>>	sheaf = NULL;                                                   
>> } else if (pcs->barn->nr_full >= MAX_FULL_SHEAVES) {                    
>>	/* racy check */                                                
>>	barn = pcs->barn;                                               
>>	keep = true;                                                    
>> }  
> 
> By the way this code also needs data_race()?

Right, will add, thanks.