Messages in this thread | | | Date | Tue, 23 Aug 2011 12:53:44 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [PATCH 13/15] x86: add cmpxchg_flag() variant |
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On 08/23/2011 12:01 PM, Christoph Lameter wrote: > On Mon, 22 Aug 2011, Jeremy Fitzhardinge wrote: > >> Most callers of cmpxchg() direcly compare RETURN with OLD to see if it was >> successful. This results in unnecessary conditional comparisons >> and conditionals since the cmpxchg instruction directly sets the flags >> to indicate success/failure. >> Add cmpxchg_flag() variants which return a boolean flag directly indicating >> success. Unfortunately an asm() statement can't directly export status >> status flags, but sete isn't too bad. > And so what happens through this patch is that a cmp with a value that is > likely in a register is replaced by a sete. Is there really a benefit?
There's definitely a benefit for functions which return a bool resulting from cmpxchg, which occurs in a few places.
> What I wish we would have is the actual use of the processor flag. > > if (cmpxchg_flags(....)) { > } > > where the cmpxchg is followed immediately by a jump. I tried in the past > to pass a goto label to cmpxchg but that did not work.
Yes, that would ideal. The closest you can get is asm goto(), but the syntax for that would be awful; something like:
#define cmpxchg_jump(ptr, old, new, fail)\ asm goto (...)
: :
again: old = *thingp; new = frobulate(old); cmpxchg_jump(thingp, old, new, again); /* worked */
Would this be useful enough?
Also, cmpxchg_flag() is limited to arch/x86; I hadn't looked into what it would take to make it a generic part of the cmpxchg API for the rest of the kernel.
J
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