Messages in this thread | | | Date | Thu, 19 Sep 2013 22:59:10 -0400 | From | Waiman Long <> | Subject | Re: [PATCH] lockref: use cmpxchg64 explicitly for lockless updates |
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On 09/19/2013 02:11 PM, Linus Torvalds wrote: > On Thu, Sep 19, 2013 at 1:06 PM, Will Deacon<will.deacon@arm.com> wrote: >> The cmpxchg() function tends not to support 64-bit arguments on 32-bit >> architectures. This could be either due to use of unsigned long arguments >> (like on ARM) or lack of instruction support (cmpxchgq on x86). However, >> these architectures may implement a specific cmpxchg64() function to >> provide 64-bit cmpxchg support instead > I'm certainly ok with this, but I wonder how much point there is to > use the cmpxchg alternatives for 32-bit architectures at all... > > From a performance standpoint, lockref really is expected to mainly > help with big machines. Only insane people would do big machines with > 32-bit kernels these days. > Of course, it may be that cmpxchg is actually faster on some > architectures, but at least on x86-32, cmpxchg8b is traditionally > quite slow. > > In other words, I'd actually like to see some numbers if there are > loads where this actually helps and matters... > > Linus
I agreed that 32-bit machines are not likely to be big enough to get benefit from this feature. However, I do see a minor problem with the current code. If a user tries to turn on CMPXCHG_LOCKREF on a 32-bit build, he/she will get a compilation error because a 64-bit data type is not supported in 32-bit mode's cmpxchg(). Because of this, my original patch also uses cmpxchg64() which is equivalent to cmpxchg() in 64-bit machine for 64-bit data type.
I would suggest either change to use cmpxchg64() or add the dependence line "depends on 64BIT" to CMPXCHG_LOCKREF.
-Longman
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