Re: [Linux-kernel-mentees] [PATCH] events: Annotate parent_ctx with __rcu

[Amol Grover <frextrite@gmail.com>]

On Mon, Feb 10, 2020 at 12:08:31PM -0500, Joel Fernandes wrote:
> On Mon, Feb 10, 2020 at 10:17:27PM +0530, Amol Grover wrote:
> > On Mon, Feb 10, 2020 at 02:34:59PM +0100, Peter Zijlstra wrote:
> > > On Mon, Feb 10, 2020 at 06:29:48PM +0530, Amol Grover wrote:
> > > > 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:
> > > 
> > > > > > @@ -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);
> 
> You can probably remove the earlier lockdep_assert_held(s) if you're going to
> use rcu_dereference_protected() here, since that would do the checking anyway.
> 

Ah yes, I was thinking this aswell.

> > > > > 
> > > > > 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?
> > > 
> > > Dunno, I've been happy just ignoring it all.
> 
> FWIW some of the sparse fixes Amol made recently did uncover so existing
> "bugs" :) (Not in perf but other code).
> 
> > > > __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.
> > > 
> > > I know what it does; what I don't know is why you need to make the code
> > > worse. In paricular, __rcu doesn't mandate rcu_dereference(), esp. not
> > > when you're actually holding the write side lock.
> > 
> > I might've misinterpreted the code. How does replacing rcu_dereference()
> > with
> > parent_ctx1 = rcu_dereference_protected(ctx1->parent_ctx,
> > 					lockdep_is_held(&ctx1->lock));
> > sound?
> 
> FWIW, some maintainers do hate calling RCU APIs when write side lock is held.
> Evidently it does make the code readability a bit worse and I can see Peter's
> point of view because the existing code is correct. I leave it to you guys to
> decide how you want to handle it.
> 

In that case, I think the code is fine as it is. Thank you for the review both!

Thanks
Amol

> thanks!
> 
>  - Joel
> 
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