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[94.245.46.49]) by smtp.gmail.com with ESMTPSA id h22sm38312ljl.96.2020.04.20.10.18.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 20 Apr 2020 10:18:38 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Mon, 20 Apr 2020 19:18:29 +0200 To: "Joel Fernandes (Google)" , Sebastian Andrzej Siewior Cc: linux-kernel@vger.kernel.org, Sebastian Andrzej Siewior , Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , "Paul E. McKenney" , rcu@vger.kernel.org, Steven Rostedt , urezki@gmail.com Subject: Re: [PATCH rcu/dev -fixes 3/4] rcu/tree: Avoid using xchg() in kfree_call_rcu_add_ptr_to_bulk() Message-ID: <20200420171829.GA12196@pc636> References: <20200420153837.194532-1-joel@joelfernandes.org> <20200420153837.194532-4-joel@joelfernandes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200420153837.194532-4-joel@joelfernandes.org> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Mon, Apr 20, 2020 at 11:38:36AM -0400, Joel Fernandes (Google) wrote: > From: Sebastian Andrzej Siewior > > There is no need to use xchg(), the access is serialized by krcp->lock. > The xchg() function adds some atomic barriers which remain hidden in > x86's disassembly but are visible on ARM for instance. > > Replace xchg() with a load + store. > > Acked-by: Joel Fernandes (Google) > Signed-off-by: Sebastian Andrzej Siewior > --- > kernel/rcu/tree.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index cd61649e1b001..f6eb3aee0935e 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -3088,7 +3088,8 @@ kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, > /* Check if a new block is required. */ > if (!krcp->bhead || > krcp->bhead->nr_records == KFREE_BULK_MAX_ENTR) { > - bnode = xchg(&krcp->bcached, NULL); > + bnode = krcp->bcached; > + krcp->bcached = NULL; > if (!bnode) { > WARN_ON_ONCE(sizeof(struct kfree_rcu_bulk_data) > PAGE_SIZE); > > But we populate the cache without holding the krcp->lock. That is why it is xchg here. See below: if (cmpxchg(&krcp->bcached, NULL, bhead)) free_page((unsigned long) bhead); we do not hold any krcp->lock here, we do not need it. -- Vlad Rezki