Re: [Qemu-devel] [PATCH 00/10, v3] target-alpha improvements

[Richard Henderson <rth@twiddle.net>]

On 03/25/2010 06:52 PM, Jamie Lokier wrote:
> I guess you are thinking to save the value loaded by load-locked,
> and use it as the "old" for host cmpxchg at target's
> store-conditional?

Yes.

> It'll be fine when ll/sc are only used to provide single-word atomic
> calculations, but I'm not sure those are the only uses to which they
> are put by any code anywhere.  E.g. if I remember rightly, there was
> some discussion of a planned unusual ll/sc use on the linux-arm
> list, which involved a second word, but the idea wasn't ever
> implemented.

"It is UNPREDICTABLE whether a processor's lock_flag is cleared by
that processor executing a normal load or store instruction."

So at least on Alpha your double-word update isn't supposed to work.

Now, what the cmpxchg *doesn't* handle is when the store isn't to
exactly the same address as the load.  The Alpha spec does allow the
store to be anywhere within the same 16-byte aligned block, and we
can't handle that.  That said, I'm pretty sure such a non-matching
store is never used on Linux.

> So stop-the-world ought to remain in, but only as a last resort to
> be used when the ll/sc sequence doesn't pass the no-funny-business
> test.

The existing stop-the-world implementation is *exactly* cmpxchg.

It doesn't do what you want: stop the world at the ll, resume the
world at the sc (or other suitable subsequent condition e.g. taken
branch or N executed instructions).


r~