Re: [PATCH bpf-next] bpf: Fix deadlock between rcu_tasks_trace and event_mutex.

[Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>]

On Tue, Feb 25, 2025 at 5:03 AM Hou Tao <houtao@xxxxxxxxxxxxxxx> wrote:
>
> Hi,
>
> On 2/25/2025 6:16 AM, Alexei Starovoitov wrote:
> > From: Alexei Starovoitov <ast@xxxxxxxxxx>
> >
> > Fix the following deadlock:
> > CPU A
> > _free_event()
> >   perf_kprobe_destroy()
> >     mutex_lock(&event_mutex)
> >       perf_trace_event_unreg()
> >         synchronize_rcu_tasks_trace()
> >
> > There are several paths where _free_event() grabs event_mutex
> > and calls sync_rcu_tasks_trace. Above is one such case.
> >
> > CPU B
> > bpf_prog_test_run_syscall()
> >   rcu_read_lock_trace()
> >     bpf_prog_run_pin_on_cpu()
> >       bpf_prog_load()
> >         bpf_tracing_func_proto()
> >           trace_set_clr_event()
> >             mutex_lock(&event_mutex)
>
> Considering the unregistered case is not so frequency, would it better
> to use mutex_trylock firstly, then fallback to workqueue when the
> event_mutex is busy ?

No. That would be an unnecessary complication.