From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Srivatsa S. Bhat" Subject: [PATCH v6 06/46] percpu_rwlock: Rearrange the read-lock code to fastpath nested percpu readers Date: Mon, 18 Feb 2013 18:09:07 +0530 Message-ID: <20130218123907.26245.37550.stgit@srivatsabhat.in.ibm.com> References: <20130218123714.26245.61816.stgit@srivatsabhat.in.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Cc: rostedt@goodmis.org, wangyun@linux.vnet.ibm.com, xiaoguangrong@linux.vnet.ibm.com, rjw@sisk.pl, sbw@mit.edu, fweisbec@gmail.com, linux@arm.linux.org.uk, nikunj@linux.vnet.ibm.com, srivatsa.bhat@linux.vnet.ibm.com, linux-pm@vger.kernel.org, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, netdev@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, walken@google.com, vincent.guittot@linaro.org To: tglx@linutronix.de, peterz@infradead.org, tj@kernel.org, oleg@redhat.com, paulmck@linux.vnet.ibm.com, rusty@rustcorp.com.au, mingo@kernel.org, akpm@linux-foundation.org, namhyung@kernel.org Return-path: In-Reply-To: <20130218123714.26245.61816.stgit@srivatsabhat.in.ibm.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org If we are dealing with a nester percpu reader, we can optimize away quite a few costly operations. Improve that fastpath further, by avoiding the unnecessary addition and subtraction of 'READER_PRESENT' to reader_refcnt, by rearranging the code a bit. Signed-off-by: Srivatsa S. Bhat --- lib/percpu-rwlock.c | 14 ++++++-------- 1 file changed, 6 insertions(+), 8 deletions(-) diff --git a/lib/percpu-rwlock.c b/lib/percpu-rwlock.c index ce7e440..ed36531 100644 --- a/lib/percpu-rwlock.c +++ b/lib/percpu-rwlock.c @@ -80,23 +80,21 @@ void percpu_read_lock_irqsafe(struct percpu_rwlock *pcpu_rwlock) preempt_disable(); /* - * Let the writer know that a reader is active, even before we choose - * our reader-side synchronization scheme. - */ - this_cpu_add(pcpu_rwlock->rw_state->reader_refcnt, READER_PRESENT); - - /* * If we are already using per-cpu refcounts, it is not safe to switch * the synchronization scheme. So continue using the refcounts. */ if (reader_uses_percpu_refcnt(pcpu_rwlock)) { this_cpu_inc(pcpu_rwlock->rw_state->reader_refcnt); - this_cpu_sub(pcpu_rwlock->rw_state->reader_refcnt, - READER_PRESENT); return; } /* + * Let the writer know that a reader is active, even before we choose + * our reader-side synchronization scheme. + */ + this_cpu_add(pcpu_rwlock->rw_state->reader_refcnt, READER_PRESENT); + + /* * The write to 'reader_refcnt' must be visible before we read * 'writer_signal'. */