Re: [PATCH v3] mm/slab: don't use kfree_rcu sheaves on PREEMPT_RT in kvfree_call_rcu()

[Vlastimil Babka (SUSE)]

On 8/31/26 16:35, Sebastian Andrzej Siewior wrote:
> On 2026-08-31 20:32:22 [+0700], ThangNN99 wrote:
>> --- a/mm/slab_common.c
>> +++ b/mm/slab_common.c
>> @@ -2034,7 +2027,14 @@ void kvfree_call_rcu(struct kvfree_rcu_head *head, void *ptr)
>>  	if (!head)
>>  		might_sleep();
>>  
>> -	if (kfree_rcu_sheaf(ptr))
>> +	/*
>> +	 * Callers may hold a raw_spinlock_t here on PREEMPT_RT (e.g.
>> +	 * set_cpus_allowed_force(), whose callers all hold
>> +	 * task_struct::pi_lock), and the sheaf/barn locks are also taken
>> +	 * as blocking locks elsewhere, so trying them here creates a
>> +	 * lockdep-visible ordering conflict. Skip sheaves on PREEMPT_RT.
>> +	 */
> 
> You mix up things. A raw_spinlock_t should work in general and should
> not cause a problem. The task_struct::pi_lock is special: It used during
> wakes and the inner waitlock of the rtmutex acquires it even during a
> trylock. For PREEMPT_RT we don't want to acquire any locks while the
> pi_lock is held.
> 
> What about
>   
>     kvfree_rcu() is called by set_cpus_allowed_force() with
>     task_struct::pi_lock acquired. On PREEMPT_RT the local_trylock()
>     usage below will acquire the waitlock which must be avoided.
>     Therefore avoid it on PREEMPT_RT.

Thanks.

> Vlastimil?

IMHO it's not useful to iterate a fix through emails with apparently a LLM
agent, so let's continue here:

https://lore.kernel.org/all/20260831-b4-kfree_rcu_hotfix-v1-1-4f0fb882638b@xxxxxxxxxx/

> 
>> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT) && kfree_rcu_sheaf(ptr))
>>  		return;
>>  
>>  	// Queue the object but don't yet schedule the batch.
> 
> Sebastian