On 6/17/26 07:14, Harry Yoo wrote: > > > 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()?
Sounds good. > 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(): Looks like the case. Well if the fallback path of kfree_nolock() that is irq_work_queue() is indeed safe here. >> /* >> * 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? >
