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

["Kumar Kartikeya Dwivedi" <memxor@gmail.com>]

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

>
> If you are OK with this change of bpf code, I will send a bpf patch
> later.
>
> Thanks,
> Tiezhu