Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 30 Jan 2020 15:44:51 +0530
From:   Amol Grover <frextrite@gmail.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        linux-kernel@vger.kernel.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>,
        "Paul E . McKenney" <paulmck@kernel.org>
Subject: Re: [PATCH 2/2] events: callchain: Use RCU API to access RCU pointer
Message-ID: <20200130101451.GA11015@workstation-portable>
References: <20200129160813.14263-1-frextrite@gmail.com>
 <20200129160813.14263-2-frextrite@gmail.com>
 <20200129221909.GA74354@google.com>
 <20200130082321.GX14879@hirez.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200130082321.GX14879@hirez.programming.kicks-ass.net>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Thu, Jan 30, 2020 at 09:23:21AM +0100, Peter Zijlstra wrote:
> 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.
> 

Got it. I'll combine them into a single patch and re-send.

> Also look at the output of:
> 
>   git log --oneline kernel/events/
> 
> and then at your $subject.
> 
> > > Signed-off-by: Amol Grover <frextrite@gmail.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@joelfernandes.org>
> 
> Do we really need that smp_read_barrier_depends() here? Then again, I
> don't suppose this is a fast path.
> 

rcu_dereference_protected is actually a lightweight API and IIRC it
omits the READ_ONCE() and hence the memory barriers.

Thanks
Amol

> 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
> > >