Re: [PATCH 5/8] mm/slab: make kfree_rcu_nolock() work with sheaves

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 4/27/26 16:45, Alexei Starovoitov wrote:
> On Mon, Apr 27, 2026 at 2:53 PM Vlastimil Babka (SUSE)
> <vbabka@kernel.org> wrote:
>> >> @@ -5973,6 +5977,12 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj)
>> >>      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 */
>> >> +                    rcu_sheaf->size--;
>> >> +                    local_unlock(&s->cpu_sheaves->lock);
>> >> +                    goto fail;
>> >> +            }
>>
>> ... this part, so an unconditional allow_spin=false on RT would only fill
>> the rcu_sheaf once and then stop working.
>>
>> However, can we condition this on in_nmi() or something, and pretend the
>> other cases of "unknown context" exist? Installing BPF hooks
>> allocating/freeing memory into call_rcu() internals sounds crazy, no?
> 
> Unfortunately no. People attach to tracepoint lock contention
> which can be inside call_rcu.
> As discussed call_rcu_nolock() is only way forward.

Alright. But we assume those tracepoint-attached callers know to use
kfree_rcu_nolock(), while kfree_rcu() callers under PREEMPT_RT can still
assume it's not called in NMI or tracepoint-attached context, right?

So I think what __kfree_rcu_sheaf() could do is
- with CONFIG_PREEMPT_RT, treat allow_spin == true as allow_spin == false
for purposes of barn_get_empty_sheaf() and to bail out before trying
alloc_empty_sheaf() - this is to avoid any lock activity that's not a trylock.
- allow_spin controls ability to do call_rcu() as above, regardless of
PREEMPT_RT, to avoid it from kfree_rcu_nolock()