Re: [PATCH v5 3/8] mm/slab: use call_rcu() in unknown context if irqs are enabled

[hu.shengming]

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+fg4rw@xxxxxxxxxxxxxx
> Suggested-by: Alexei Starovoitov <ast@xxxxxxxxxx>
> Signed-off-by: Harry Yoo (Oracle) <harry@xxxxxxxxxx>
> ---
>  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 <hu.shengming@xxxxxxxxxx>

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