On Wed, May 15, 2013 at 05:48:47PM +0200, Paolo Bonzini wrote:
> This emulates Win32 manual-reset events using futexes or conditional
> variables. Typical ways to use them are with multi-producer,
> single-consumer data structures, to test for a complex condition whose
> elements come from different threads:
>
> for (;;) {
> qemu_event_reset(ev);
> ... test complex condition ...
> if (condition is true) {
> break;
> }
> qemu_event_wait(ev);
> }
>
> Alternatively:
>
> ... compute condition ...
> if (condition) {
> do {
> qemu_event_wait(ev);
> qemu_event_reset(ev);
> ... compute condition ...
> } while(condition);
> qemu_event_set(ev);
> }
>
> QemuEvent provides a very fast userspace path in the common case when
> no other thread is waiting, or the event is not changing state. It
> is used to report RCU quiescent states to the thread calling
> synchronize_rcu (the latter being the single consumer), and to report
> call_rcu invocations to the thread that receives them.
It would be nice to describe the need for the Linux futex code. pthread
mutex/condvars are implemented in terms of futexes already, so how much
benefit is there - I thought they stay in userspace in the non-contended
case too?
Stefan
