Re: [PATCH] events: Annotate parent_ctx with __rcu

[Amol Grover <frextrite@gmail.com>]

On Mon, Feb 10, 2020 at 10:36:24AM +0100, Peter Zijlstra wrote:
> On Sat, Feb 08, 2020 at 08:16:49PM +0530, Amol Grover wrote:
> > parent_ctx is used under RCU context in kernel/events/core.c,
> > tell sparse about it aswell.
> > 
> > Fixes the following instances of sparse error:
> > kernel/events/core.c:3221:18: error: incompatible types in comparison
> > kernel/events/core.c:3222:23: error: incompatible types in comparison
> > 
> > This introduces the following two sparse errors:
> > kernel/events/core.c:3117:18: error: incompatible types in comparison
> > kernel/events/core.c:3121:30: error: incompatible types in comparison
> > 
> > Hence, use rcu_dereference() to access parent_ctx and silence
> > the newly introduced errors.
> > 
> > Signed-off-by: Amol Grover <frextrite@gmail.com>
> > ---
> >  include/linux/perf_event.h |  2 +-
> >  kernel/events/core.c       | 11 ++++++++---
> >  2 files changed, 9 insertions(+), 4 deletions(-)
> > 
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index 6d4c22aee384..e18a7bdc6f98 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -810,7 +810,7 @@ struct perf_event_context {
> >  	 * These fields let us detect when two contexts have both
> >  	 * been cloned (inherited) from a common ancestor.
> >  	 */
> > -	struct perf_event_context	*parent_ctx;
> > +	struct perf_event_context __rcu	*parent_ctx;
> >  	u64				parent_gen;
> >  	u64				generation;
> >  	int				pin_count;
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 2173c23c25b4..2a8c5670b254 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -3106,26 +3106,31 @@ static void ctx_sched_out(struct perf_event_context *ctx,
> >  static int context_equiv(struct perf_event_context *ctx1,
> >  			 struct perf_event_context *ctx2)
> >  {
> > +	struct perf_event_context *parent_ctx1, *parent_ctx2;
> > +
> >  	lockdep_assert_held(&ctx1->lock);
> >  	lockdep_assert_held(&ctx2->lock);
> >  
> > +	parent_ctx1 = rcu_dereference(ctx1->parent_ctx);
> > +	parent_ctx2 = rcu_dereference(ctx2->parent_ctx);
> 
> Bah.
> 
> Why are you  fixing all this sparse crap and making the code worse?

Hi Peter,

Sparse is quite noisy and we need to eliminate false-positives, right?
__rcu will tell the developer, this pointer could change and he needs to
take the required steps to make sure the code doesn't break.

Thanks
Amol