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

[Tiezhu Yang]

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.

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

Thanks,
Tiezhu