Re: [PATCH v1] LoongArch: Fix __smp_mb__{before,after}_atomic()

On 2026/8/18 下午9:35, Kumar Kartikeya Dwivedi wrote:
> On Tue Aug 18, 2026 at 3:28 PM CEST, Tiezhu Yang wrote:
>> On 2026/8/17 上午11:59, Tiezhu Yang wrote:
>>> When testing the BPF selftest "sudo ./test_progs -t timer_lockup", there
>>> is a kernel lockup and panic:
>>
>> ...
>>
>>> With this patch, the lockless "store-before-load" ordering is enforced by
>>> the DBAR instruction. The BPF timer_lockup selftest was stressed for 5000
>>> consecutive loops on a physical LoongArch machine without encountering any
>>> further lockups or warnings:
>>>
>>>     for i in {1..5000}; do sudo ./test_progs -t timer_lockup; done
>>
>> ...
>>
>>> diff --git a/arch/loongarch/include/asm/barrier.h b/arch/loongarch/include/asm/barrier.h
>>> index 4b663f197706..adfe343dfa65 100644
>>> --- a/arch/loongarch/include/asm/barrier.h
>>> +++ b/arch/loongarch/include/asm/barrier.h
>>> @@ -57,8 +57,8 @@
>>>    #define __WEAK_LLSC_MB		"			\n"
>>>    #endif
>>>
>>> -#define __smp_mb__before_atomic()	barrier()
>>> -#define __smp_mb__after_atomic()	barrier()
>>> +#define __smp_mb__before_atomic()	__smp_mb()
>>> +#define __smp_mb__after_atomic()	__smp_mb()
>>>
>>>    /**
>>>     * array_index_mask_nospec() - generate a ~0 mask when index < size, 0 otherwise
>>
>> Hi all,
>>
>> To address the soft lockup while avoiding the performance overhead of
>> executing two separate instructions, I think there is a more elegant
>> way to fix this directly in the BPF core helper:
>>
>> We can replace atomic_inc() and smp_mb__after_atomic() with a single
>> atomic_fetch_add() in bpf_timer_cancel(). This consolidates the logic
>> into a single native atomic operation with full ordering, which not
>> only guarantees the store-load order to eliminate the deadlock but
>> also improves the performance for weak memory model architectures.
>>
>> Any thoughts on this approach?
>>
>> ```
>> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
>> index c18f1e16edee..ee2b3a4dcc05 100644
>> --- a/kernel/bpf/helpers.c
>> +++ b/kernel/bpf/helpers.c
>> @@ -1591,9 +1591,7 @@ BPF_CALL_1(bpf_timer_cancel, struct bpf_async_kern
>> *, async)
>>            */
>>           if (!cur_t)
>>                   goto drop;
>> -       atomic_inc(&t->cancelling);
>> -       /* Need full barrier after relaxed atomic_inc */
>> -       smp_mb__after_atomic();
>> +       atomic_fetch_add(1, &t->cancelling);
>>           inc = true;
>>           if (atomic_read(&cur_t->cancelling)) {
>>                   /* We're cancelling timer t, while some other timer
>> callback is
>> ```
>>
>> I tested the above diff, the BPF timer_lockup selftest was stressed
>> for 5000 consecutive loops on a physical LoongArch machine without
>> encountering any further lockups or warnings.
> 
> There is a full barrier in both cases. I don't know what performance improvement
> will be achieved by changing this.

We performed benchmarking using UnixBench on a physical LoongArch
machine, the UnixBench score is different (amadd.w + dbar < amadd_db.w).

> Don't you need to fix the lowering for
> smp_mb__after_atomic() anyway? It's used in several other places in the kernel.

The LoongArch architecture-level fix for smp_mb__after_atomic() is
a fundamental bug fix that will be pushed separately to the LoongArch
tree. This BPF-layer change is intended as a generic, cross-architecture
performance optimization.

Thanks,
Tiezhu