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authorAndrea Parri <parri.andrea@gmail.com>2018-02-20 19:45:56 +0100
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2018-05-30 07:50:28 +0200
commitbffff2e16f50ba5e6cbe4e5ccdb2e478965de06d (patch)
treed22638193c77efb4b1759404faee7c63435f66b9 /arch/alpha
parentbe4132e07364c5a758ac9d0c8e89dbf663f41f78 (diff)
locking/xchg/alpha: Add unconditional memory barrier to cmpxchg()
[ 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>
Diffstat (limited to 'arch/alpha')
-rw-r--r--arch/alpha/include/asm/xchg.h15
1 files changed, 7 insertions, 8 deletions
diff --git a/arch/alpha/include/asm/xchg.h b/arch/alpha/include/asm/xchg.h
index 0ca9724597c1..d0a1cf4c7010 100644
--- a/arch/alpha/include/asm/xchg.h
+++ b/arch/alpha/include/asm/xchg.h
@@ -127,10 +127,9 @@ ____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
@@ -149,8 +148,8 @@ ____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"
@@ -176,8 +175,8 @@ ____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"
@@ -199,8 +198,8 @@ ____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"
@@ -222,8 +221,8 @@ ____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"