Re: [PATCH v3] perf/core: Fix reentry problem in perf_output_read_group

[Mark Rutland]

On Tue, Aug 16, 2022 at 05:31:27PM +0200, Peter Zijlstra wrote:
> On Tue, Aug 16, 2022 at 03:54:19PM +0100, Mark Rutland wrote:
> > > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > > index ee8b9ecdc03b..d4d53b9ba71e 100644
> > > --- a/include/linux/perf_event.h
> > > +++ b/include/linux/perf_event.h
> > > @@ -631,7 +631,12 @@ struct pmu_event_list {
> > >  	struct list_head	list;
> > >  };
> > >  
> > > +/*
> > > + * Iterating the sibling list requires this list to be stable; by ensuring IRQs
> > > + * are disabled IPIs from perf_{install_in,remove_from}_context() are held off.
> > > + */
> > >  #define for_each_sibling_event(sibling, event)			\
> > > +	lockdep_assert_irqs_disabled();				\
> > >  	if ((event)->group_leader == (event))			\
> > >  		list_for_each_entry((sibling), &(event)->sibling_list, sibling_list)
> > >  
> > 
> > I had a go with v6.0-rc1 and Vince's perf fuzzer immediately triggered a bunch
> > of cases (dump below).
> > 
> > I had thought holding the context mutex protected some of these cases, even
> > with IRQs unmasked?
> 
> Ah yes.. duh. How's this then?
> 
> 
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index ee8b9ecdc03b..4d9cf508c510 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -631,7 +631,21 @@ struct pmu_event_list {
>  	struct list_head	list;
>  };
>  
> +#ifdef CONFIG_LOCKDEP
> +#define LOCKDEP_ASSERT_EVENT_CTX(event)				\
> +	WARN_ON_ONCE(__lockdep_enabled &&			\
> +		     (this_cpu_read(hardirqs_enabled) ||	\
> +		      lockdep_is_held(&(event)->ctx->mutex) != LOCK_STATE_HELD))

Uh, should that `||` be `&&`, or am I missing the plot?

This'll warn if IRQs are enabled regardless of whether the mutex is held.

Thanks,
Mark.

> +#else
> +#define LOCKDEP_ASSERT_EVENT_CTX(event)
> +#endif
> +
> +/*
> + * Iterating the sibling list requires this list to be stable; by ensuring IRQs
> + * are disabled IPIs from perf_{install_in,remove_from}_context() are held off.
> + */
>  #define for_each_sibling_event(sibling, event)			\
> +	LOCKDEP_ASSERT_EVENT_CTX(event);			\
>  	if ((event)->group_leader == (event))			\
>  		list_for_each_entry((sibling), &(event)->sibling_list, sibling_list)
>