Re: [PATCH slab/for-next v4 5/8] mm/slab: allow kfree_rcu_sheaf() on PREEMPT_RT

[Harry Yoo <harry@kernel.org>]

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On 7/21/26 7:46 PM, Vlastimil Babka (SUSE) wrote:
> On 7/20/26 14:44, Harry Yoo (Oracle) wrote:
>> As suggested by Vlastimil Babka [1], kfree_rcu_sheaf() can be used
>> on PREEMPT_RT if we always assume spinning is not allowed on PREEMPT_RT.
>> This is because local_trylock and spinlock_t are safe to use with
>> trylock and unlock as long as the kernel does not spin and the context
>> is not NMI and not hardirq.
>>
>> Now that __kfree_rcu_sheaf() knows how to handle SLAB_FREE_NOLOCK,
>> relax the limitation and try the sheaves path on PREEMPT_RT as well.
>>
>> Keep the lockdep map on non RT kernels. However, do not use the lockdep
>> map on PREEMPT_RT to avoid suppressing valid lockdep warnings.
>>
>> As pointed by Vlastimil Babka [2], on PREEMPT_RT it is unnecessary to
>> defer call_rcu() under IRQ-disabled section or raw spinlock. However,
>> let us avoid adding more complexity as the scenario is not supposed
>> to be common on PREEMPT_RT, with a hope that call_rcu_nolock() will be
>> soon supported in RCU.
>>
>> Link: https://lore.kernel.org/linux-mm/6811cc17-8ee4-48c8-8cbf-6bf4d9f98162@kernel.org [1]
>> Link: https://lore.kernel.org/linux-mm/40591888-3a87-433e-b3d2-cda1cab543be@kernel.org [2]
>> Suggested-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
>> Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>
> 
> Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

Thanks!

> Nit:
> 
>> ---
>>  mm/slab_common.c | 12 ++++++++++--
>>  mm/slub.c        | 17 ++++++++++-------
>>  2 files changed, 20 insertions(+), 9 deletions(-)
>>
>> diff --git a/mm/slab_common.c b/mm/slab_common.c
>> index 9c9ae1384f47..8c631bf97cd5 100644
>> --- a/mm/slab_common.c
>> +++ b/mm/slab_common.c
>> @@ -1595,6 +1595,14 @@ static bool kfree_rcu_sheaf(void *obj)
>>  {
>>  	struct kmem_cache *s;
>>  	struct slab *slab;
>> +	unsigned int free_flags = SLAB_FREE_DEFAULT;
>> +
>> +	/*
>> +	 * It is not safe to spin on PREEMPT_RT because the kernel might be
>> +	 * holding a raw spinlock and slab acquires sleeping locks.
>> +	 */
>> +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
>> +		free_flags = SLAB_FREE_NOLOCK;
>>  
>>  	if (is_vmalloc_addr(obj))
>>  		return false;
>> @@ -1605,7 +1613,7 @@ static bool kfree_rcu_sheaf(void *obj)
>>  
>>  	s = slab->slab_cache;
>>  	if (likely(!IS_ENABLED(CONFIG_NUMA) || slab_nid(slab) == numa_mem_id()))
>> -		return __kfree_rcu_sheaf(s, obj, SLAB_FREE_DEFAULT);
>> +		return __kfree_rcu_sheaf(s, obj, free_flags);
>>  
>>  	return false;
>>  }
>> @@ -1954,7 +1962,7 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr)
>>  	if (!head)
>>  		might_sleep();
>>  
>> -	if (!IS_ENABLED(CONFIG_PREEMPT_RT) && kfree_rcu_sheaf(ptr))
>> +	if (kfree_rcu_sheaf(ptr))
>>  		return;
>>  
>>  	// Queue the object but don't yet schedule the batch.
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 8afa6b47b1f2..deac315d0f23 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -6065,12 +6065,13 @@ static void rcu_free_sheaf(struct rcu_head *head)
>>   * kvfree_call_rcu() can be called while holding a raw_spinlock_t. Since
>>   * __kfree_rcu_sheaf() may acquire a spinlock_t (sleeping lock on PREEMPT_RT),
>>   * this would violate lock nesting rules. Therefore, kvfree_call_rcu() avoids
>> - * this problem by bypassing the sheaves layer entirely on PREEMPT_RT.
>> + * this problem by passing allow_spin = false on PREEMPT_RT.
> 
> by passing SLAB_FREE_NOLOCK ?

Ouch, I noticed this but forgot to update.
Will adjust, thanks.

-- 
Cheers,
Harry / Hyeonggon

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