linux-kernel - [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Pine.LNX.4.64.1210221936570.25995@file.rdu.redhat.com>
Date:	Mon, 22 Oct 2012 19:37:47 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <anton@...hat.com>, linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers

This patch introduces new barrier pair light_mb() and heavy_mb() for
percpu rw semaphores.

This patch fixes a bug in percpu-rw-semaphores where a barrier was
missing in percpu_up_write.

This patch improves performance on the read path of
percpu-rw-semaphores: on non-x86 cpus, there was a smp_mb() in
percpu_up_read. This patch changes it to a compiler barrier and removes
the "#if defined(X86) ..." condition.

From: Lai Jiangshan <laijs@...fujitsu.com>
Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>

---
 include/linux/percpu-rwsem.h |   20 +++++++-------------
 1 file changed, 7 insertions(+), 13 deletions(-)

Index: linux-3.6.3-fast/include/linux/percpu-rwsem.h
===================================================================
--- linux-3.6.3-fast.orig/include/linux/percpu-rwsem.h	2012-10-22 23:37:57.000000000 +0200
+++ linux-3.6.3-fast/include/linux/percpu-rwsem.h	2012-10-23 01:21:23.000000000 +0200
@@ -12,6 +12,9 @@ struct percpu_rw_semaphore {
 	struct mutex mtx;
 };
 
+#define light_mb()	barrier()
+#define heavy_mb()	synchronize_sched()
+
 static inline void percpu_down_read(struct percpu_rw_semaphore *p)
 {
 	rcu_read_lock();
@@ -24,22 +27,12 @@ static inline void percpu_down_read(stru
 	}
 	this_cpu_inc(*p->counters);
 	rcu_read_unlock();
+	light_mb(); /* A, between read of p->locked and read of data, paired with D */
 }
 
 static inline void percpu_up_read(struct percpu_rw_semaphore *p)
 {
-	/*
-	 * On X86, write operation in this_cpu_dec serves as a memory unlock
-	 * barrier (i.e. memory accesses may be moved before the write, but
-	 * no memory accesses are moved past the write).
-	 * On other architectures this may not be the case, so we need smp_mb()
-	 * there.
-	 */
-#if defined(CONFIG_X86) && (!defined(CONFIG_X86_PPRO_FENCE) && !defined(CONFIG_X86_OOSTORE))
-	barrier();
-#else
-	smp_mb();
-#endif
+	light_mb(); /* B, between read of the data and write to p->counter, paired with C */
 	this_cpu_dec(*p->counters);
 }
 
@@ -61,11 +54,12 @@ static inline void percpu_down_write(str
 	synchronize_rcu();
 	while (__percpu_count(p->counters))
 		msleep(1);
-	smp_rmb(); /* paired with smp_mb() in percpu_sem_up_read() */
+	heavy_mb(); /* C, between read of p->counter and write to data, paired with B */
 }
 
 static inline void percpu_up_write(struct percpu_rw_semaphore *p)
 {
+	heavy_mb(); /* D, between write to data and write to p->locked, paired with A */
 	p->locked = false;
 	mutex_unlock(&p->mtx);
 }

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/