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McKenney" , Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Thomas Gleixner , Mike Galbraith Subject: Re: [PATCH 3/3] rcu: Avoid using xchg() in kfree_call_rcu_add_ptr_to_bulk() Message-ID: <20200420152351.GG176663@google.com> References: <20200415160034.662274-1-bigeasy@linutronix.de> <20200415160034.662274-4-bigeasy@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200415160034.662274-4-bigeasy@linutronix.de> Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Wed, Apr 15, 2020 at 06:00:34PM +0200, Sebastian Andrzej Siewior wrote: > 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. > > 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 b31b61721a9ff..04d11c49ff3b9 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -3079,7 +3079,8 @@ 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; Acked-by: Joel Fernandes (Google) I'll resend this with a rewrite of your 1/3 based on the lates tdiscussions. For your 2/3, let us hold off on that for now as my rewrite of 1/3 will require rewriting your 2/3. thanks, - Joel > + krcp->bcached = NULL; > if (!bnode) { > WARN_ON_ONCE(sizeof(struct kfree_rcu_bulk_data) > PAGE_SIZE); > > -- > 2.26.0 >