Re: [PATCH] tools/memory-model: remove rb-dep, smp_read_barrier_depends, and lockless_dereference

From: Alan Stern
Date: Tue Feb 20 2018 - 10:38:56 EST


On Tue, 20 Feb 2018, Andrea Parri wrote:

> > This leaves us with a question: Do we want to change the kernel by
> > adding memory barriers after unsuccessful RMW operations on Alpha, or
> > do we want to change the model by excluding such operations from
> > address dependencies?
>
> 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?

Yes, this seems reasonable to me. If Will gives it his "Acked-by"
to go with Peter's, you should submit it to Ingo Molnar.

And once this is made, there shouldn't be any trouble with the proposed
patch for the memory model.

Alan

> 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"
>
>
> >
> > Note that operations like atomic_add_unless() already include memory
> > barriers.
> >
> > Alan