Re: [PATCH bpf-next 2/3] bpf: Avoid deadlock using trylock when popping LRU free nodes

[Leon Hwang <leon.hwang@linux.dev>]

On 20/1/26 02:46, bot+bpf-ci@kernel.org wrote:
>> diff --git a/kernel/bpf/bpf_lru_list.c b/kernel/bpf/bpf_lru_list.c
>> index c091f3232cc5..03d37f72731a 100644
>> --- a/kernel/bpf/bpf_lru_list.c
>> +++ b/kernel/bpf/bpf_lru_list.c
> 
> [ ... ]
> 
>> @@ -451,13 +455,12 @@ static struct bpf_lru_node *bpf_common_lru_pop_free(struct bpf_lru *lru,
>>
>>  	loc_l = per_cpu_ptr(clru->local_list, cpu);
>>
>> -	raw_spin_lock_irqsave(&loc_l->lock, flags);
>> +	if (!raw_spin_trylock_irqsave(&loc_l->lock, flags))
>> +		return NULL;
>>
>>  	node = __local_list_pop_free(loc_l);
>> -	if (!node) {
>> -		bpf_lru_list_pop_free_to_local(lru, loc_l);
>> +	if (!node && bpf_lru_list_pop_free_to_local(lru, loc_l))
>>  		node = __local_list_pop_free(loc_l);
>> -	}
>>
>>  	if (node)
>>  		__local_list_add_pending(lru, loc_l, cpu, node, hash);
> 
> The trylock conversion here looks correct, but what about the steal path
> that follows later in bpf_common_lru_pop_free()? Looking at the existing
> code (not shown in this diff), the steal loop still uses unconditional
> raw_spin_lock_irqsave():
> 
>     do {
>         steal_loc_l = per_cpu_ptr(clru->local_list, steal);
> 
>         raw_spin_lock_irqsave(&steal_loc_l->lock, flags);
>         ...
>     } while (...);
> 
> If the goal is to avoid NMI-context deadlocks when acquiring LRU locks,
> can the same deadlock scenario occur when NMI interrupts during the steal
> loop and the NMI handler tries to acquire the same steal_loc_l->lock?
> 
> Similarly, after a successful steal, there is another unconditional lock:
> 
>     if (node) {
>         raw_spin_lock_irqsave(&loc_l->lock, flags);
>         __local_list_add_pending(lru, loc_l, cpu, node, hash);
>         raw_spin_unlock_irqrestore(&loc_l->lock, flags);
>     }
> 
> Should these also use trylock to maintain consistency with the stated goal
> of avoiding NMI-context deadlocks?
> 

This patch is not intended to eliminate all possible deadlock scenarios.
Its goal is to avoid deadlocks caused by long-lived critical sections
in the free-node pop paths, where lock contention can persist and lead
to re-entrant lock acquisition from NMI context.

The steal path and the post-steal update are both short-lived critical
sections. They do not exhibit the same contention characteristics and
have not been observed to trigger the reported deadlock scenarios.
Converting these paths to trylock would add complexity without clear
benefit, and is therefore unnecessary for the stated goal of this change.

Thanks,
Leon

> 
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
> 
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/21147913717