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

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Mathieu Desnoyers wrote:
* Nick Piggin ([email protected]) 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.
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