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.and...@gmail.com> Acked-by: Peter Zijlstra <pet...@infradead.org> Cc: Will Deacon <will.dea...@arm.com> Cc: "Paul E. McKenney" <paul...@linux.vnet.ibm.com> Cc: Alan Stern <st...@rowland.harvard.edu> Cc: Richard Henderson <r...@twiddle.net> Cc: Ivan Kokshaysky <i...@jurassic.park.msu.ru> Cc: Matt Turner <matts...@gmail.com> Cc: linux-al...@vger.kernel.org Cc: linux-kernel@vger.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
