Re: [PATCH RFC 1/2] arch: Introduce ARCH_HAS_HW_XCHG_SMALL

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jul 27, 2021 at 01:03:32AM +0800, Boqun Feng wrote:

> I'm missing you point here, a) ll/sc can provide forward progress and b)
> ll/sc instructions are used to implement xchg/cmpxchg (see ARM64 and
> PPC).

Correct on both counts, but b) is tricky, even if a), then it doesn't
hold that the primitive resulting from b) also provides fwd progress.

I feel this point is often overlooked. I should go add something to
atomic_t.txt about that I suppose.

> > How to make CPU guarantee  "load + cmpxchg" forward-progress? Fusion
> > these instructions and lock the snoop channel?
> > Maybe hardware guys would think that it's easier to implement cas +
> > dcas + amo(short & byte).
> > 
> 
> Please note that if _Q_PENDING_BITS == 1, then the xchg_tail() is
> implemented as a "load + cmpxchg", so if "load + cmpxchg" implementation
> of xchg16() doesn't provide forward-progress in an architecture, neither
> does xchg_tail().

Right, so generally we rely on cmpxchg() to provide fairness. Some
architectures (notably Sparc64) go to great lengths to ensure this.

I have memories of adding backoff to an LL/SC based arch at some point,
but I cannot find it in a hurry, so it could be one of the since deleted
archs.