Commit-ID: cb13b424e986aed68d74cbaec3449ea23c50e167
Gitweb: https://git.kernel.org/tip/cb13b424e986aed68d74cbaec3449ea23c50e167
Author: Andrea Parri <parri.andrea@xxxxxxxxx>
AuthorDate: Tue, 20 Feb 2018 19:45:56 +0100
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
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@xxxxxxxxx>
Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Cc: Ivan Kokshaysky <ink@xxxxxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Matt Turner <mattst88@xxxxxxxxx>
Cc: Richard Henderson <rth@xxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: linux-alpha@xxxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/1519152356-4804-1-git-send-email-parri.andrea@xxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
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"
--
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