[tip:ras/core] x86/mce: Remove explicit smp_rmb() when starting CPUs sync

[tip-bot for Davidlohr Bueso]

Commit-ID:  bf92b1feb658f6a262daf3a87d790997a1dca0ff
Gitweb:     http://git.kernel.org/tip/bf92b1feb658f6a262daf3a87d790997a1dca0ff
Author:     Davidlohr Bueso <dave@xxxxxxxxxxxx>
AuthorDate: Wed, 6 Apr 2016 10:05:15 +0200
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 13 Apr 2016 10:54:23 +0200

x86/mce: Remove explicit smp_rmb() when starting CPUs sync

mce_start() has an explicit smp_wmb() to serialize writes to global_nwo
and mce_callin. However, atomic_inc_return() implies barriers on both
sides of the call, as such simply rely on this full SMP barrier.

Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: linux-edac <linux-edac@xxxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1458602396-840-1-git-send-email-dave@xxxxxxxxxxxx
Link: http://lkml.kernel.org/r/1459929916-12852-3-git-send-email-bp@xxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/kernel/cpu/mcheck/mce.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
index f0c921b..6b7039c 100644
--- a/arch/x86/kernel/cpu/mcheck/mce.c
+++ b/arch/x86/kernel/cpu/mcheck/mce.c
@@ -830,9 +830,9 @@ static int mce_start(int *no_way_out)
 
 	atomic_add(*no_way_out, &global_nwo);
 	/*
-	 * global_nwo should be updated before mce_callin
+	 * Rely on the implied barrier below, such that global_nwo
+	 * is updated before mce_callin.
 	 */
-	smp_wmb();
 	order = atomic_inc_return(&mce_callin);
 
 	/*