netdev - [PATCH v6 06/46] percpu_rwlock: Rearrange the read-lock code to fastpath nested percpu readers

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Message-ID: <20130218123907.26245.37550.stgit@srivatsabhat.in.ibm.com>
Date:	Mon, 18 Feb 2013 18:09:07 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	tglx@...utronix.de, peterz@...radead.org, tj@...nel.org,
	oleg@...hat.com, paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org
Cc:	rostedt@...dmis.org, wangyun@...ux.vnet.ibm.com,
	xiaoguangrong@...ux.vnet.ibm.com, rjw@...k.pl, sbw@....edu,
	fweisbec@...il.com, linux@....linux.org.uk,
	nikunj@...ux.vnet.ibm.com, srivatsa.bhat@...ux.vnet.ibm.com,
	linux-pm@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	walken@...gle.com, vincent.guittot@...aro.org
Subject: [PATCH v6 06/46] percpu_rwlock: Rearrange the read-lock code to
 fastpath nested percpu readers

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 <srivatsa.bhat@...ux.vnet.ibm.com>
---

 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'.
 	 */

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