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

2018-02-21 Thread tip-bot for Andrea Parri 
Commit-ID:  cb13b424e986aed68d74cbaec3449ea23c50e167
Gitweb: https://git.kernel.org/tip/cb13b424e986aed68d74cbaec3449ea23c50e167
Author: Andrea Parri 
AuthorDate: Tue, 20 Feb 2018 19:45:56 +0100
Committer:  Ingo Molnar 
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=150884953419377=2
https://marc.info/?l=linux-kernel=150884946319353=2
https://marc.info/?l=linux-kernel=151215810824468=2
https://marc.info/?l=linux-kernel=151215816324484=2

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

Signed-off-by: Andrea Parri 
Acked-by: Peter Zijlstra 
Acked-by: Paul E. McKenney 
Cc: Alan Stern 
Cc: Ivan Kokshaysky 
Cc: Linus Torvalds 
Cc: Matt Turner 
Cc: Richard Henderson 
Cc: Thomas Gleixner 
Cc: Will Deacon 
Cc: linux-al...@vger.kernel.org
Link: 
http://lkml.kernel.org/r/1519152356-4804-1-git-send-email-parri.and...@gmail.com
Signed-off-by: Ingo Molnar 
---
 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"

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

2018-02-21 Thread tip-bot for Andrea Parri 
Commit-ID:  cb13b424e986aed68d74cbaec3449ea23c50e167
Gitweb: https://git.kernel.org/tip/cb13b424e986aed68d74cbaec3449ea23c50e167
Author: Andrea Parri 
AuthorDate: Tue, 20 Feb 2018 19:45:56 +0100
Committer:  Ingo Molnar 
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=150884953419377=2
https://marc.info/?l=linux-kernel=150884946319353=2
https://marc.info/?l=linux-kernel=151215810824468=2
https://marc.info/?l=linux-kernel=151215816324484=2

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

Signed-off-by: Andrea Parri 
Acked-by: Peter Zijlstra 
Acked-by: Paul E. McKenney 
Cc: Alan Stern 
Cc: Ivan Kokshaysky 
Cc: Linus Torvalds 
Cc: Matt Turner 
Cc: Richard Henderson 
Cc: Thomas Gleixner 
Cc: Will Deacon 
Cc: linux-al...@vger.kernel.org
Link: 
http://lkml.kernel.org/r/1519152356-4804-1-git-send-email-parri.and...@gmail.com
Signed-off-by: Ingo Molnar 
---
 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"