Re: [PATCH bpf-next v5 2/3] perf: Add atomic operation in get_recursion_context

[Tao Chen]

在 2025/11/10 16:52, Peter Zijlstra 写道:
> On Mon, Nov 10, 2025 at 12:35:58AM +0800, Tao Chen wrote:
> >  From BPF side, preemption usually is enabled. Yonghong said, it is
> > possible that both tasks (at process level) may reach right before
> > "recursion[rctx]++;". In such cases, both tasks will be able to get
> > buffer and this is not right.
> >
> > Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
> > Signed-off-by: Tao Chen <chen.dylane@xxxxxxxxx>
> > ---
>
> Nope, this function really is meant to be used with preemption disabled.
> If BPF doesn't abide, fix that.

Ok, let us use preempt_disable in bpf stackmap, thanks. I will change it 
in v6.

> >   kernel/events/internal.h | 5 +----
> >   1 file changed, 1 insertion(+), 4 deletions(-)
> >
> > diff --git a/kernel/events/internal.h b/kernel/events/internal.h
> > index d9cc5708309..684bde972ba 100644
> > --- a/kernel/events/internal.h
> > +++ b/kernel/events/internal.h
> > @@ -214,12 +214,9 @@ static inline int get_recursion_context(u8 *recursion)
> >   {
> >   	unsigned char rctx = interrupt_context_level();
> >   
> > -	if (recursion[rctx])
> > +	if (cmpxchg(&recursion[rctx], 0, 1) != 0)
> >   		return -1;
> >   
> > -	recursion[rctx]++;
> > -	barrier();
> > -
> >   	return rctx;
> >   }
> >   
> > --
> > 2.48.1


--
Best Regards
Tao Chen