Re: [PATCH for-next v3 6/9] mm/slab: allow kfree_rcu_sheaf() on PREEMPT_RT

On 6/17/26 2:24 AM, Vlastimil Babka (SUSE) wrote:
> On 6/15/26 13:06, Harry Yoo (Oracle) wrote:
>> As suggested by Vlastimil Babka, 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 variant 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 allow_spin = false,
>> 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.
>>
>> Link: https://lore.kernel.org/linux-mm/[email protected]
>> Suggested-by: Vlastimil Babka (SUSE) <[email protected]>
>> Signed-off-by: Harry Yoo (Oracle) <[email protected]>
> 
> LGTM, but maybe unnecessary pessimistic wrt call_rcu() on PREEMPT_RT?
> I thought (in the Link: above) we'd only need to downgrade allow_spin to
> false on PREEMPT_RT for handling sheaves movement from/to barn and
> alloc_empty_sheaf(), but call_rcu() would be safe from kfree_rcu() even on
> RT?

Indeed. Good point, thanks!

Hmm, but I'm not sure that it's worth the complexity given that
PREEMPT_RT tries very hard to avoid disabling IRQs...

> Or is the irqs_disabled() condition rare enough so we don't care?

Given that most users don't call kfree_rcu() under raw spinlock or
IRQs-disabled section on PREEMPT_RT, I think it's okay to keep it as is
(it's not making things worse, at least) and wait for call_rcu_nolock()?

On a side note, I don't have much idea on what needs to call kfree_rcu()
under a raw spinlock, other than set_cpus_allowed_force(), which should
really be using kfree_nolock() instead of kfree_rcu() once we support
kmalloc() -> kfree_nolock():
> 	/*
> 	 * Because this is called with p->pi_lock held, it is not possible
> 	 * to use kfree() here (when PREEMPT_RT=y), therefore punt to using
> 	 * kfree_rcu().
> 	 */
> 	kfree_rcu((union cpumask_rcuhead *)ac.user_mask, rcu);

Any thoughts, RCU/RT folks?

-- 
Cheers,
Harry / Hyeonggon

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