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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