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

Boqun Feng
2015-10-13T14:59:01Z

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On Tue, Oct 13, 2015 at 03:43:33PM +0100, Will Deacon wrote:
> On Tue, Oct 13, 2015 at 10:32:59PM +0800, Boqun Feng wrote:
[snip]
> >
> > 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.
>
> I didn't think this was ambiguous... A failed cmpxchg_release doesn't
> perform a store, so because the RELEASE semantics only apply to the
> store portion of the operation, it therefore doesn't have any ordering
> guarantees. Acquire is called out as a special case because it *does*
> actually perform a load on the failure case.
>

Make sense.

> > 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..
>
> Putting a barrier in the middle of that critical section is probably a
> terrible idea, and that's why I thought you were avoiding it (hence my

The fact is that I haven't thought of that way to implement
cmpxchg_release before you ask that question ;-) And I'm not going to do
that for now and probably not in the future.

> original question). Perhaps just add a comment to that effect, since I

Are you suggesting if I put a barrier in the middle I'd better to add a
comment, right? So if I don't do that, it's OK to let this patch as it.

Regards,
Boqun

> fear adding more words to memory-barriers.txt is just likely to create
> further confusion.
>
> Will

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