Re: [PATCH v2] perf/core: Fix data race between perf_event_set_output and perf_mmap_close

[Yang Jihong]

Hello,

On 2022/7/5 21:07, Peter Zijlstra wrote:
> On Mon, Jul 04, 2022 at 05:26:04PM +0200, Peter Zijlstra wrote:
> > On Mon, Jul 04, 2022 at 08:00:06PM +0800, Yang Jihong wrote:
> > > Data race exists between perf_event_set_output and perf_mmap_close.
> > > The scenario is as follows:
> > >
> > >                    CPU1                                                       CPU2
> > >                                                                      perf_mmap_close(event2)
> > >                                                                        if (atomic_dec_and_test(&event2->rb->mmap_count)  // mmap_count 1 -> 0
> > >                                                                          detach_rest = true;
> > > ioctl(event1, PERF_EVENT_IOC_SET_OUTPUT, event2)
> > >    perf_event_set_output(event1, event2)
> > >                                                                        if (!detach_rest)
> > >                                                                          goto out_put;
> > >                                                                        list_for_each_entry_rcu(event, &event2->rb->event_list, rb_entry)
> > >                                                                          ring_buffer_attach(event, NULL)
> > >                                                                        // because event1 has not been added to event2->rb->event_list,
> > >                                                                        // event1->rb is not set to NULL in these loops
> > >
> > >      ring_buffer_attach(event1, event2->rb)
> > >        list_add_rcu(&event1->rb_entry, &event2->rb->event_list)
> > >
> > > The above data race causes a problem, that is, event1->rb is not NULL, but event1->rb->mmap_count is 0.
> > > If the perf_mmap interface is invoked for the fd of event1, the kernel keeps in the perf_mmap infinite loop:
> > >
> > > again:
> > >          mutex_lock(&event->mmap_mutex);
> > >          if (event->rb) {
> > > <SNIP>
> > >                  if (!atomic_inc_not_zero(&event->rb->mmap_count)) {
> > >                          /*
> > >                           * Raced against perf_mmap_close() through
> > >                           * perf_event_set_output(). Try again, hope for better
> > >                           * luck.
> > >                           */
> > >                          mutex_unlock(&event->mmap_mutex);
> > >                          goto again;
> > >                  }
> > > <SNIP>
> >
> > Too tired, must look again tomorrow, little feeback below.
>
> With brain more awake I ended up with the below. Does that work?

Yes, I apply the patch on kernel versions 5.10 and mainline,
and it could fixed the problem.

Tested-by: Yang Jihong <yangjihong1@xxxxxxxxxx>

Thanks,
Yang