Re: [PATCH linux-next] perf/ring_buffer: Add EPOLLRDNORM flag for poll
From: Ingo Molnar
Date: Thu Mar 13 2025 - 13:10:07 EST
* Tao Chen <chen.dylane@xxxxxxxxx> wrote:
> 在 2025/3/13 18:05, Ingo Molnar 写道:
> >
> > * Tao Chen <chen.dylane@xxxxxxxxx> wrote:
> >
> > > The poll man page says POLLRDNORM is equivalent to POLLIN,
> > > so add EPOLLRDNORM here.
> > >
> > > Signed-off-by: Tao Chen <chen.dylane@xxxxxxxxx>
> > > ---
> > > kernel/events/ring_buffer.c | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> > > index 59a52b1a1..5130b119d 100644
> > > --- a/kernel/events/ring_buffer.c
> > > +++ b/kernel/events/ring_buffer.c
> > > @@ -19,7 +19,7 @@
> > > static void perf_output_wakeup(struct perf_output_handle *handle)
> > > {
> > > - atomic_set(&handle->rb->poll, EPOLLIN);
> > > + atomic_set(&handle->rb->poll, EPOLLIN | EPOLLRDNORM);
> >
> > So what does EPOLLRDNORM mean to begin with? There doesn't seem to be
> > separate/specific handling of it anywhere in the kernel that I can
> > see...
> >
>
> It seems that if user set pollfd with POLLRDNORM, perf_poll will not return
> until timeout even if perf_output_wakeup called, whereas POLLIN returns.
Mind adding this to the changelog, and explain that this patch fixes
this particular poll() functionality and semantics for userspace?
Thanks,
Ingo