Harry wrote: > On Thu, Jul 30, 2026 at 07:21:19PM +0800, [email protected] wrote: > > Harry wrote: > > > call_rcu() disables IRQs with local_irq_save() to protect its per-cpu > > > data structures. Therefore, if IRQs are not disabled, they cannot be > > > corrupted by reentrance into call_rcu(). So fall back to the deferred > > > path only when !allow_spin && irqs_disabled(). > > > > > > The RCU subsystem does not guarantee this contractually, and this > > > optimization relies on RCU's implementation details. Ideally, it should > > > be removed once call_rcu_nolock() is supported by the RCU subsystem. > > > > > > Link: > > > https://lore.kernel.org/linux-mm/caadnvqkrvd5zsnekbzzu7w86ghbghuug2pvzpgztngns+fg...@mail.gmail.com > > > Suggested-by: Alexei Starovoitov <[email protected]> > > > Signed-off-by: Harry Yoo (Oracle) <[email protected]> > > > --- > > > mm/slub.c | 8 ++++++-- > > > 1 file changed, 6 insertions(+), 2 deletions(-) > > > > > > diff --git a/mm/slub.c b/mm/slub.c > > > index 2c5ba8c326e7..bb0f2975c95b 100644 > > > --- a/mm/slub.c > > > +++ b/mm/slub.c > > > @@ -6157,8 +6157,12 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void > > > *obj, unsigned int free_flags) > > > if (likely(rcu_sheaf->size < s->sheaf_capacity)) { > > > rcu_sheaf = NULL; > > > } else { > > > - if (unlikely(!allow_spin)) { > > > - /* call_rcu() cannot be called in an unknown context */ > > > + /* > > > + * With !allow_spin, we might have interrupted call_rcu()'s > > > + * IRQ-disabled critical section. If IRQs are not disabled, > > > + * we know that's not the case. > > > + */ > > > + if (unlikely(!allow_spin && irqs_disabled())) { > > > > LGTM! > > Reviewed-by: Shengming Hu <[email protected]> > > Thanks a lot, Shengming! >
I'm glad to have the opportunity to learn from your work! > > I initially wondered whether in_nmi() should also be checked here. > > However, irqs_disabled() already covers the relevant reentrancy case, > > since call_rcu() disables IRQs while modifying its per-CPU data. > > Exactly, that was my reasoning ;) > > As a side note there is an attempt to do this in call_rcu() instead of > slab: > > https://lore.kernel.org/all/[email protected] > > Just FYI. Thanks for sharing this update. It's exciting to see the RCU subsystem continuing to evolve and gain broader support. -- With Best Regards, Shengming
