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

[Vlastimil Babka]

On 1/13/26 1:31 PM, Harry Yoo wrote:
> On Tue, Jan 13, 2026 at 10:32:33AM +0100, Vlastimil Babka wrote:
>> 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@xxxxxxxxx>
>>>> Closes: https://lore.kernel.org/oe-lkp/202601121442.c530bed3-lkp@xxxxxxxxx
>>>> Fixes: 0f35040de593 ("mm/slab: introduce kvfree_rcu_barrier_on_cache() for cache destruction")
>>>> Cc: stable@xxxxxxxxxxxxxxx
>>>> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
>>>> ---
>>>
>>> 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.
> 
> I thought about it a little bit more...
> 
> It's not because a cpu slab was installed again (for list_slab_objects()
> to be called on a slab, it must be on n->partial list), but because

Hmm that's true.

> flush_slab() cannot handle concurrent frees to the cpu slab.
> 
> CPU X                                CPU Y
> 
> - flush_slab() reads
>   c->freelist
>                                      rcu_free_sheaf_nobarn()
> 				     ->sheaf_flush_unused()
> 				     ->__kmem_cache_free_bulk()
> 				     ->do_slab_free()
> 				       -> sees slab == c->slab
> 				       -> frees to c->freelist
> - c->slab = NULL,
>   c->freelist = NULL
> - call deactivate_slab()
>   ^ the object freed by sheaf_flush_unused() is leaked,
>     thus slab->inuse != 0

But for this to be the same "c" it has to be the same cpu, not different
X and Y, no?
And that case is protected I think, the action by X with
local_lock_irqsave() prevents an irq handler to execute Y. Action Y is
using __update_cpu_freelist_fast to find out it was interrupted by X
messing with c-> fields.


> That said, flush_slab() works fine only when it is guaranteed that
> there will be no concurrent frees to the cpu slab (acquiring local_lock
> in flush_slab() doesn't help because free fastpath doesn't take it)
> 
> calling rcu_barrier() before flush_all_cpus_locked() ensures
> there will be no concurrent frees.
> 
> A side question; I'm not sure how __kmem_cache_shrink(),
> validate_slab_cache(), cpu_partial_store() are supposed to work
> correctly? They call flush_all() without guaranteeing there will be
> no concurrent frees to the cpu slab.
> 
> ...probably doesn't matter after sheaves-for-all :)
> 
>> 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?
> 
> Agreed. As it's called (after flushing rcu sheaves) in
> kvfree_rcu_barrier_on_cache(), it's not necessary in
> __kmem_cache_shutdown().
> 
>>>>  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.
>