Re: [PATCH RFC v2 01/20] mm/slab: add rcu_barrier() to kvfree_rcu_barrier_on_cache()

[Vlastimil Babka <vbabka@suse.cz>]

On 1/13/26 3:08 AM, Harry Yoo wrote:
> On Mon, Jan 12, 2026 at 04:16:55PM +0100, Vlastimil Babka wrote:
>> After we submit the rcu_free sheaves to call_rcu() we need to make sure
>> the rcu callbacks complete. kvfree_rcu_barrier() does that via
>> flush_all_rcu_sheaves() but kvfree_rcu_barrier_on_cache() doesn't. Fix
>> that.
> 
> Oops, my bad.
> 
>> Reported-by: kernel test robot <oliver.sang@intel.com>
>> Closes: https://lore.kernel.org/oe-lkp/202601121442.c530bed3-lkp@intel.com
>> Fixes: 0f35040de593 ("mm/slab: introduce kvfree_rcu_barrier_on_cache() for cache destruction")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>> ---
> 
> The fix looks good to me, but I wonder why
> `if (s->sheaf_capacity) rcu_barrier();` in __kmem_cache_shutdown()
> didn't prevent the bug from happening?

Hmm good point, didn't notice it's there.

I think it doesn't help because it happens only after
flush_all_cpus_locked(). And the callback from rcu_free_sheaf_nobarn()
will do sheaf_flush_unused() and end up installing the cpu slab again.

Because the bot flagged commit "slab: add sheaves to most caches" where
cpu slabs still exist. It's thus possible that with the full series, the
bug is gone. But we should prevent it upfront anyway. The rcu_barrier()
in __kmem_cache_shutdown() however is probably unnecessary then and we
can remove it, right?

>>  mm/slab_common.c | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/slab_common.c b/mm/slab_common.c
>> index eed7ea556cb1..ee994ec7f251 100644
>> --- a/mm/slab_common.c
>> +++ b/mm/slab_common.c
>> @@ -2133,8 +2133,11 @@ EXPORT_SYMBOL_GPL(kvfree_rcu_barrier);
>>   */
>>  void kvfree_rcu_barrier_on_cache(struct kmem_cache *s)
>>  {
>> -	if (s->cpu_sheaves)
>> +	if (s->cpu_sheaves) {
>>  		flush_rcu_sheaves_on_cache(s);
>> +		rcu_barrier();
>> +	}
>> +
>>  	/*
>>  	 * TODO: Introduce a version of __kvfree_rcu_barrier() that works
>>  	 * on a specific slab cache.
>>
>> -- 
>> 2.52.0
>>
>