Re: futex atomic vs ordering constraints

[Chris Metcalf <cmetcalf@ezchip.com>]

On 09/02/2015 08:55 AM, Peter Zijlstra wrote:
> So here goes..
>
> Chris, I'm awfully sorry, but I seem to be Tile challenged.
>
> TileGX seems to define:
>
> #define smp_mb__before_atomic()	smp_mb()
> #define smp_mb__after_atomic()	smp_mb()
>
> However, its atomic_add_return() implementation looks like:
>
> static inline int atomic_add_return(int i, atomic_t *v)
> {
> 	int val;
> 	smp_mb();  /* barrier for proper semantics */
> 	val = __insn_fetchadd4((void *)&v->counter, i) + i;
> 	barrier();  /* the "+ i" above will wait on memory */
> 	return val;
> }
>
> Which leaves me confused on smp_mb__after_atomic().

Are you concerned about whether it has proper memory
barrier semantics already, i.e. full barriers before and after?
In fact we do have a full barrier before, but then because of the
"+ i" / "barrier()", we know that the only other operation since
the previous mb(), namely the read of v->counter, has
completed after the atomic operation.  As a result we can
omit explicitly having a second barrier.

It does seem like all the current memory-order semantics are
correct, unless I'm missing something!

> That said, your futex ops seem to lack any memory barrier, so naively
> I'd add both, its just that your add_return() confuses me.

So something like this?

diff --git a/arch/tile/include/asm/futex.h b/arch/tile/include/asm/futex.h
index 1a6ef1b69cb1..0a5501b11d02 100644
--- a/arch/tile/include/asm/futex.h
+++ b/arch/tile/include/asm/futex.h
@@ -39,6 +39,7 @@
  #ifdef __tilegx__
  
  #define __futex_asm(OP) \
+	smp_mb();						\
  	asm("1: {" #OP " %1, %3, %4; movei %0, 0 }\n"		\
  	    ".pushsection .fixup,\"ax\"\n"			\
  	    "0: { movei %0, %5; j 9f }\n"			\
@@ -48,7 +49,8 @@
  	    ".popsection\n"					\
  	    "9:"						\
  	    : "=r" (ret), "=r" (val), "+m" (*(uaddr))		\
-	    : "r" (uaddr), "r" (oparg), "i" (-EFAULT))
+	    : "r" (uaddr), "r" (oparg), "i" (-EFAULT)); 	\
+	smp_mb()
  
  #define __futex_set() __futex_asm(exch4)
  #define __futex_add() __futex_asm(fetchadd4)
@@ -75,7 +77,10 @@
  
  #define __futex_call(FN)						\
  	{								\
-		struct __get_user gu = FN((u32 __force *)uaddr, lock, oparg); \
+		struct __get_user gu;					\
+		smp_mb();						\
+		gu = FN((u32 __force *)uaddr, lock, oparg);		\
+		/* See smp_mb__after_atomic() */			\
  		val = gu.val;						\
  		ret = gu.err;						\
  	}

-- 
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com