linux-kernel - [PATCH tip/core/rcu 1/4] mce: Stop using array-index-based RCU primitives

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Message-Id: <1431465524-32246-1-git-send-email-paulmck@linux.vnet.ibm.com>
Date:	Tue, 12 May 2015 14:18:41 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	bobby.prani@...il.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-edac@...r.kernel.org, Tony Luck <tony.luck@...el.com>,
	Borislav Petkov <bp@...en8.de>
Subject: [PATCH tip/core/rcu 1/4] mce: Stop using array-index-based RCU primitives

From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>

Because mce is arch-specific x86 code, there is little or no
performance benefit of using rcu_dereference_index_check() over using
smp_load_acquire().  It also turns out that mce is the only place that
array-index-based RCU is used, and it would be convenient to drop
this portion of the RCU API.

This patch therefore changes rcu_dereference_index_check() uses to
smp_load_acquire(), but keeping the lockdep diagnostics, and also
changes rcu_access_index() uses to READ_ONCE().

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: linux-edac@...r.kernel.org
Cc: Tony Luck <tony.luck@...el.com>
Cc: Borislav Petkov <bp@...en8.de>
---
 arch/x86/kernel/cpu/mcheck/mce.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
index e535533d5ab8..1fd04a02d90a 100644
--- a/arch/x86/kernel/cpu/mcheck/mce.c
+++ b/arch/x86/kernel/cpu/mcheck/mce.c
@@ -53,9 +53,12 @@
 static DEFINE_MUTEX(mce_chrdev_read_mutex);
 
 #define rcu_dereference_check_mce(p) \
-	rcu_dereference_index_check((p), \
-			      rcu_read_lock_sched_held() || \
-			      lockdep_is_held(&mce_chrdev_read_mutex))
+({ \
+	rcu_lockdep_assert(rcu_read_lock_sched_held() || \
+			   lockdep_is_held(&mce_chrdev_read_mutex), \
+			   "suspicious rcu_dereference_check_mce() usage"); \
+	smp_load_acquire(&(p)); \
+})
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/mce.h>
@@ -1884,7 +1887,7 @@ out:
 static unsigned int mce_chrdev_poll(struct file *file, poll_table *wait)
 {
 	poll_wait(file, &mce_chrdev_wait, wait);
-	if (rcu_access_index(mcelog.next))
+	if (READ_ONCE(mcelog.next))
 		return POLLIN | POLLRDNORM;
 	if (!mce_apei_read_done && apei_check_mce())
 		return POLLIN | POLLRDNORM;
-- 
1.8.1.5

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