linux-kernel - [tip:locking/urgent] locking/xchg/alpha: Add unconditional memory barrier to cmpxchg()

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Date:   Wed, 21 Feb 2018 02:49:57 -0800
From:   tip-bot for Andrea Parri <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     mingo@...nel.org, tglx@...utronix.de, stern@...land.harvard.edu,
        torvalds@...ux-foundation.org, parri.andrea@...il.com,
        rth@...ddle.net, hpa@...or.com, mattst88@...il.com,
        linux-kernel@...r.kernel.org, peterz@...radead.org,
        ink@...assic.park.msu.ru, will.deacon@....com,
        paulmck@...ux.vnet.ibm.com
Subject: [tip:locking/urgent] locking/xchg/alpha: Add unconditional memory
 barrier to cmpxchg()

Commit-ID:  cb13b424e986aed68d74cbaec3449ea23c50e167
Gitweb:     https://git.kernel.org/tip/cb13b424e986aed68d74cbaec3449ea23c50e167
Author:     Andrea Parri <parri.andrea@...il.com>
AuthorDate: Tue, 20 Feb 2018 19:45:56 +0100
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 21 Feb 2018 10:12:29 +0100

locking/xchg/alpha: Add unconditional memory barrier to cmpxchg()

Continuing along with the fight against smp_read_barrier_depends() [1]
(or rather, against its improper use), add an unconditional barrier to
cmpxchg.  This guarantees that dependency ordering is preserved when a
dependency is headed by an unsuccessful cmpxchg.  As it turns out, the
change could enable further simplification of LKMM as proposed in [2].

[1] https://marc.info/?l=linux-kernel&m=150884953419377&w=2
    https://marc.info/?l=linux-kernel&m=150884946319353&w=2
    https://marc.info/?l=linux-kernel&m=151215810824468&w=2
    https://marc.info/?l=linux-kernel&m=151215816324484&w=2

[2] https://marc.info/?l=linux-kernel&m=151881978314872&w=2

Signed-off-by: Andrea Parri <parri.andrea@...il.com>
Acked-by: Peter Zijlstra <peterz@...radead.org>
Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Alan Stern <stern@...land.harvard.edu>
Cc: Ivan Kokshaysky <ink@...assic.park.msu.ru>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Matt Turner <mattst88@...il.com>
Cc: Richard Henderson <rth@...ddle.net>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Will Deacon <will.deacon@....com>
Cc: linux-alpha@...r.kernel.org
Link: http://lkml.kernel.org/r/1519152356-4804-1-git-send-email-parri.andrea@gmail.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/alpha/include/asm/xchg.h | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/arch/alpha/include/asm/xchg.h b/arch/alpha/include/asm/xchg.h
index 68dfb3c..e266086 100644
--- a/arch/alpha/include/asm/xchg.h
+++ b/arch/alpha/include/asm/xchg.h
@@ -128,10 +128,9 @@ ____xchg(, volatile void *ptr, unsigned long x, int size)
  * store NEW in MEM.  Return the initial value in MEM.  Success is
  * indicated by comparing RETURN with OLD.
  *
- * The memory barrier should be placed in SMP only when we actually
- * make the change. If we don't change anything (so if the returned
- * prev is equal to old) then we aren't acquiring anything new and
- * we don't need any memory barrier as far I can tell.
+ * The memory barrier is placed in SMP unconditionally, in order to
+ * guarantee that dependency ordering is preserved when a dependency
+ * is headed by an unsuccessful operation.
  */
 
 static inline unsigned long
@@ -150,8 +149,8 @@ ____cmpxchg(_u8, volatile char *m, unsigned char old, unsigned char new)
 	"	or	%1,%2,%2\n"
 	"	stq_c	%2,0(%4)\n"
 	"	beq	%2,3f\n"
-		__ASM__MB
 	"2:\n"
+		__ASM__MB
 	".subsection 2\n"
 	"3:	br	1b\n"
 	".previous"
@@ -177,8 +176,8 @@ ____cmpxchg(_u16, volatile short *m, unsigned short old, unsigned short new)
 	"	or	%1,%2,%2\n"
 	"	stq_c	%2,0(%4)\n"
 	"	beq	%2,3f\n"
-		__ASM__MB
 	"2:\n"
+		__ASM__MB
 	".subsection 2\n"
 	"3:	br	1b\n"
 	".previous"
@@ -200,8 +199,8 @@ ____cmpxchg(_u32, volatile int *m, int old, int new)
 	"	mov %4,%1\n"
 	"	stl_c %1,%2\n"
 	"	beq %1,3f\n"
-		__ASM__MB
 	"2:\n"
+		__ASM__MB
 	".subsection 2\n"
 	"3:	br 1b\n"
 	".previous"
@@ -223,8 +222,8 @@ ____cmpxchg(_u64, volatile long *m, unsigned long old, unsigned long new)
 	"	mov %4,%1\n"
 	"	stq_c %1,%2\n"
 	"	beq %1,3f\n"
-		__ASM__MB
 	"2:\n"
+		__ASM__MB
 	".subsection 2\n"
 	"3:	br 1b\n"
 	".previous"