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Message-Id: <1519152356-4804-1-git-send-email-parri.andrea@gmail.com>
Date: Tue, 20 Feb 2018 19:45:56 +0100
From: Andrea Parri <parri.andrea@...il.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: Andrea Parri <parri.andrea@...il.com>,
Will Deacon <will.deacon@....com>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Alan Stern <stern@...land.harvard.edu>,
Richard Henderson <rth@...ddle.net>,
Ivan Kokshaysky <ink@...assic.park.msu.ru>,
Matt Turner <mattst88@...il.com>, linux-alpha@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: [PATCH] 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>
Cc: Will Deacon <will.deacon@....com>
Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: Alan Stern <stern@...land.harvard.edu>
Cc: Richard Henderson <rth@...ddle.net>
Cc: Ivan Kokshaysky <ink@...assic.park.msu.ru>
Cc: Matt Turner <mattst88@...il.com>
Cc: linux-alpha@...r.kernel.org
Cc: linux-kernel@...r.kernel.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 68dfb3cb71454..e2660866ce972 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"
--
2.7.4
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