* 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. Mathieu -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
