Michael Schnell wrote: > Jim Donelson wrote: > > All that is really required is an atomic exchange. > > Suppose 1 means taken, 0 means free. I do an exchange with a 1. If I got > > back a zero, it's mine. > > True with a Mutex, not true with a Futex. Here you need a second bit in > user-space that tells the releaser that it is to wake up a sleeping > waiter. Otherwise, any release would need a system call.
Indeed. A atomic-swap instruction provides enough bits, but it's hard to get the algorithm right. > Seemingly the best instruction to handle these bits is "atomic compare > and exchange" (e.g. provided by the X86 CPU). It's actually not the best, because when it returns "did not match" you have to loop and try again. The amount of looping depends on contention level. Ideal would be an atomic-do-what-I-need-for-my-algorithm operation. However, atomic-compare-exchange can be used for everything, so it's a useful instruction if you have to pick one. -- 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
