linux-kernel - Re: [PATCH 2/2] events: callchain: Use RCU API to access RCU pointer

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Date:   Thu, 30 Jan 2020 09:23:21 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Amol Grover <frextrite@...il.com>, 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,
        Madhuparna Bhowmik <madhuparnabhowmik04@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH 2/2] events: callchain: Use RCU API to access RCU pointer

On Wed, Jan 29, 2020 at 05:19:09PM -0500, Joel Fernandes wrote:
> On Wed, Jan 29, 2020 at 09:38:13PM +0530, Amol Grover wrote:
> > callchain_cpus_entries is annotated as an RCU pointer.
> > Hence rcu_dereference_protected or similar RCU API is
> > required to dereference the pointer.
> > 
> > This fixes the following sparse warning
> > kernel/events/callchain.c:65:17: warning: incorrect type in assignment

Seems silly to have this two patches; the first introduces the second
issue, might as well fix it all in one go.

Also look at the output of:

  git log --oneline kernel/events/

and then at your $subject.

> > Signed-off-by: Amol Grover <frextrite@...il.com>
> > ---
> >  kernel/events/callchain.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
> > index f91e1f41d25d..a672d02a1b3a 100644
> > --- a/kernel/events/callchain.c
> > +++ b/kernel/events/callchain.c
> > @@ -62,7 +62,8 @@ static void release_callchain_buffers(void)
> >  {
> >  	struct callchain_cpus_entries *entries;
> >  
> > -	entries = callchain_cpus_entries;
> > +	entries = rcu_dereference_protected(callchain_cpus_entries,
> > +					    lockdep_is_held(&callchain_mutex));
> 
> 
> Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>

Do we really need that smp_read_barrier_depends() here? Then again, I
don't suppose this is a fast path.

IIRC even Alpha got the dependent write ordering right.

> >  	RCU_INIT_POINTER(callchain_cpus_entries, NULL);
> >  	call_rcu(&entries->rcu_head, release_callchain_buffers_rcu);
> >  }
> > -- 
> > 2.24.1
> >