Re: [PATCH v3 6/6] powerpc: atomic: Implement cmpxchg{,64}_* and atomic{,64}_cmpxchg_* variants

[Boqun Feng <boqun.feng@gmail.com>]

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On Tue, Oct 13, 2015 at 02:24:04PM +0100, Will Deacon wrote:
> On Mon, Oct 12, 2015 at 10:14:06PM +0800, Boqun Feng wrote:
> > Implement cmpxchg{,64}_relaxed and atomic{,64}_cmpxchg_relaxed, based on
> > which _release variants can be built.
> > 
> > To avoid superfluous barriers in _acquire variants, we implement these
> > operations with assembly code rather use __atomic_op_acquire() to build
> > them automatically.
> 
> The "superfluous barriers" are for the case where the cmpxchg fails, right?

Yes.

> And you don't do the same thing for release, because you want to avoid a
> barrier in the middle of the critical section?
> 

Mostly because of the comments in include/linux/atomic.h:

 * For compound atomics performing both a load and a store, ACQUIRE
 * semantics apply only to the load and RELEASE semantics only to the
 * store portion of the operation. Note that a failed cmpxchg_acquire
 * does -not- imply any memory ordering constraints.

so I thought only the barrier in cmpxchg_acquire() is conditional, and
the barrier in cmpxchg_release() is not. Maybe we'd better call it out
that cmpxchg *family* doesn't have any order guarantee if cmp fails, as
a complement of

ed2de9f74ecb ("locking/Documentation: Clarify failed cmpxchg() memory ordering semantics")

Because it seems this commit only claims that the barriers in fully
ordered version are conditional.


If cmpxchg_release doesn't have order guarantee when failed, I guess I
can implement it with a barrier in the middle as you mentioned:

	unsigned int prev;

	__asm__ __volatile__ (
"1:	lwarx	%0,0,%2		
	cmpw	0,%0,%3\n\
	bne-	2f\n"
	PPC_RELEASE_BARRIER
"	stwcx.	%4,0,%2\n\
	bne-	1b"
	"\n\
2:"
	: "=&r" (prev), "+m" (*p)
	: "r" (p), "r" (old), "r" (new)
	: "cc", "memory");

	return prev;


However, I need to check whether the architecture allows this and any
other problem exists.

Besides, I don't think it's a good idea to do the "put barrier in the
middle" thing in this patchset, because that seems a premature
optimization and if we go further, I guess we can also replace the
PPC_RELEASE_BARRIER above with a "sync" to implement a fully ordered
version cmpxchg(). Too much needs to investigate then..

> (just checking I understand your reasoning).
> 

That actually helps me find a probably better implementation if allowed,
thank you ;-)

Regards,
Boqun

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