On Tue, Aug 11, 2009 at 4:58 PM, Jamie Lokier <ja...@shareable.org> wrote:
> Jim Donelson wrote: > > I'd like to see the code for compare_exchange and the lock function. > > compare_exchange is a single architecture-specific instruction; that's > what we're discussing. Lock functions are described in Ulrich > Drepper's futex paper. In the context of Drepper's paper, you code looks bugged. You add, not subtract, and you never set mem to 0. > > > > The purpose of a spin lock is to avoid a more expensive kernel call > > > if the mutex is released quickly (or not taken at all). Presumably > > > you enter the kernel after n tries and sleep so that you are not > > > using up quanta while spinning. > > > > That doesn't work with a single processor. While you are spinning, > > it's impossible for the other thread to release the mutex until you > > are preempted, so potential benefit from spinning is marginal and > > often outweight by the benefit of not spinning. > > > > Of course it does - sleeping on a sp means "preempt me now". > > Yes, but *spinning* on a spinlock does not. You propose spinning n > times before sleeping. The mutex will only be released during spin > time if you are randomly preempted during that short time. Obviously, in the case of an SP, n = 1. > > > -- Jamie > _______________________________________________ > uClinux-dev mailing list > uClinux-dev@uclinux.org > http://mailman.uclinux.org/mailman/listinfo/uclinux-dev > This message was resent by uclinux-dev@uclinux.org > To unsubscribe see: > http://mailman.uclinux.org/mailman/options/uclinux-dev >
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