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 <[email protected]>
Acked-by: Peter Zijlstra <[email protected]>
Acked-by: Paul E. McKenney <[email protected]>
Cc: Alan Stern <[email protected]>
Cc: Ivan Kokshaysky <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Matt Turner <[email protected]>
Cc: Richard Henderson <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

diff --git a/arch/alpha/include/asm/xchg.h b/arch/alpha/include/asm/xchg.h
index 68dfb3cb71454384dd187edfd0dded0fe4b65117..e2660866ce9722986d13a51f49472f789bba59da 100644 (file)
--- 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"