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McKenney" Subject: Re: [PATCH 2/2] events: callchain: Use RCU API to access RCU pointer Message-ID: <20200130133248.GW14914@hirez.programming.kicks-ass.net> References: <20200129160813.14263-1-frextrite@gmail.com> <20200129160813.14263-2-frextrite@gmail.com> <20200129221909.GA74354@google.com> <20200130082321.GX14879@hirez.programming.kicks-ass.net> <20200130101451.GA11015@workstation-portable> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200130101451.GA11015@workstation-portable> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 30, 2020 at 03:44:51PM +0530, Amol Grover wrote: > > > > 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) > > > > 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. Oh argh, indeed. I suppose I should've had more tea this morning.