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Subject[PATCH 4.9 124/329] locking/xchg/alpha: Add unconditional memory barrier to cmpxchg()
Date
4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Andrea Parri <parri.andrea@gmail.com>

[ Upstream commit cb13b424e986aed68d74cbaec3449ea23c50e167 ]

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@gmail.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-alpha@vger.kernel.org
Link: http://lkml.kernel.org/r/1519152356-4804-1-git-send-email-parri.andrea@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/alpha/include/asm/xchg.h | 15 +++++++--------
1 file changed, 7 insertions(+), 8 deletions(-)

--- a/arch/alpha/include/asm/xchg.h
+++ b/arch/alpha/include/asm/xchg.h
@@ -127,10 +127,9 @@ ____xchg(, volatile void *ptr, unsigned
* 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
@@ -149,8 +148,8 @@ ____cmpxchg(_u8, volatile char *m, unsig
" 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"
@@ -176,8 +175,8 @@ ____cmpxchg(_u16, volatile short *m, uns
" 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"
@@ -199,8 +198,8 @@ ____cmpxchg(_u32, volatile int *m, int o
" 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"
@@ -222,8 +221,8 @@ ____cmpxchg(_u64, volatile long *m, unsi
" 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"

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