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]> 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. The reasoning looks sound to me : ) -- With Best Regards, Shengming
