On Tue, Feb 20, 2018 at 10:33:47AM +0100, Andrea Parri wrote: > I'd like to continue to treat R[once] and R*[once] equally if possible. > Given the (unconditional) smp_read_barrier_depends in READ_ONCE and in > atomics, it seems reasonable to have it unconditionally in cmpxchg. > > As with the following patch? > > Andrea > > --- > 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"
ACK
