Re: [patch 01/28] Fall back on interrupt disable in cmpxchg8b on 80386 and 80486

[Nick Piggin <nickpiggin@yahoo.com.au>]

Mathieu Desnoyers wrote:
> * Nick Piggin (nickpiggin@yahoo.com.au) wrote:
>
> > Mathieu Desnoyers wrote:
> >
> >
> > > Q:
> > > What's the reason to have cmpxchg64_local on 32 bit architectures?
> > > Without that need all this would just be a few simple defines.
> > >
> > > A:
> > > cmpxchg64_local on 32 bits architectures takes unsigned long long
> > > parameters, but cmpxchg_local only takes longs. Since we have cmpxchg8b
> > > to execute a 8 byte cmpxchg atomically on pentium and +, it makes sense
> > > to provide a flavor of cmpxchg and cmpxchg_local using this instruction.
> > >
> > > Also, for 32 bits architectures lacking the 64 bits atomic cmpxchg, it
> > > makes sense _not_ to define cmpxchg64 while cmpxchg could still be
> > > available.
> > >
> > > Moreover, the fallback for cmpxchg8b on i386 for 386 and 486 is a
> > > different case than cmpxchg (which is only required for 386). Using
> > > different code makes this easier.
> > >
> > > However, cmpxchg64_local will be emulated by disabling interrupts on all
> > > architectures where it is not supported atomically.
> > >
> > > Therefore, we *could* turn cmpxchg64_local into a cmpxchg_local, but it
> > > would make the 386/486 fallbacks ugly, make its design different from
> > > cmpxchg/cmpxchg64 (which really depends on atomic operations and cannot
> > > be emulated) and require the __cmpxchg_local to be expressed as a macro
> > > rather than an inline function so the parameters would not be fixed to
> > > unsigned long long in every case.
> > >
> > > So I think cmpxchg64_local makes sense there, but I am open to
> > > suggestions.
> >
> > Every new thing like this (especially 64 bit operation on 32 bit
> > architectures) adds a tiny bit more burden for maintainers. Are
> > there any callers? If not, don't add it. It's simple to add if we
> > do get a good reason.
>
>
> I am actually using it in LTTng in my timestamping code. I use it to
> work around CPUs with asynchronous TSCs. I need to update 64 bits
> values atomically on this 32 bits architecture.
>
> I plan to submit this timestamping code soon.

OK fair enough. So long as there is a user (and you are sure said
user is going to get upstream -- sometimes it is easier to put
this patchset in with the one that is going to call it, but OTOH
that can turn people off reviewing).

--
SUSE Labs, Novell Inc.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/