4.9-stable review patch. If anyone has any objections, please let me know.
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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... 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"