Re: [PATCH v3 06/11] perf: Add support for SIGTRAP on perf events

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Mar 25, 2021 at 09:14:39AM +0100, Marco Elver wrote:
> On Wed, Mar 24, 2021 at 12:24PM +0100, Marco Elver wrote:
> [...]
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index b6434697c516..1e4c949bf75f 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -6391,6 +6391,17 @@ void perf_event_wakeup(struct perf_event *event)
> >  	}
> >  }
> >  
> > +static void perf_sigtrap(struct perf_event *event)
> > +{
> > +	struct kernel_siginfo info;
> > +
> 
> I think we need to add something like this here:
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 4b82788fbaab..4fcd6b45ce66 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6395,6 +6395,13 @@ static void perf_sigtrap(struct perf_event *event)
>  {
>  	struct kernel_siginfo info;
>  
> +	/*
> +	 * This irq_work can race with an exiting task; bail out if sighand has
> +	 * already been released in release_task().
> +	 */
> +	if (!current->sighand)
> +		return;
> +
>  	clear_siginfo(&info);
>  	info.si_signo = SIGTRAP;
>  	info.si_code = TRAP_PERF;
> 
> 

Urgh.. I'm not entirely sure that check is correct, but I always forget
the rules with signal. It could be we ought to be testing PF_EXISTING
instead.

But also, I think Jiri Olsa was going to poke around here because all of
this is broken on PREEMPT_RT. IIRC the plan was to add yet another stage
to the construct. So where today we have:


	<NMI>
		irq_work_queue()
	</NMI>
	...
	<IRQ>
		perf_pending_event()
	</IRQ>

(and we might already have a problem on some architectures where there
can be significant time between these due to not having
arch_irq_work_raise(), so ideally we ought to double check current in
your case)

The idea was, I think to add a task_work(), such that we get:

	<NMI>
		irq_work_queue()
	</NMI>
	...
	<IRQ>
		perf_pending_event()
		  task_work_add()
	</IRQ>

	<ret-to-user>
		run_task_work()
		  ...
		    kill_fasync();