linux-kernel - Re: [PATCH] events: Annotate parent_ctx with _

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Date:   Mon, 10 Feb 2020 14:34:59 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Amol Grover <frextrite@...il.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        Joel Fernandes <joel@...lfernandes.org>,
        Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH] events: Annotate parent_ctx with __rcu

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);
> > 
> > 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.

> __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.