Re: [Qemu-devel] [PATCH v5 11/13] cpu-exec-common: Introduce async_safe_run_on_cpu()

[Paolo Bonzini <pbonzini@redhat.com>]

> The problem with CPUs coming up late is indeed present in this patch,
> I'll review your patch on the flight. :)
> 
> synchronize_rcu() is actually relatively cheap with URCU, so I guess
> that's fine.  An alternative to that could be a pthread_barrier_t, but
> it can be added later.
> 
> Another way to fix the issue with a variable number of waiters could be
> to wrap safe work with rcu_read_lock and rcu_read_unlock, and put a
> synchronize_rcu() at the beginning of the CPU thread function.  But it
> can be done later too.
> 
> Your patch from a year ago, right now, seems to be the best to me.  I'd
> like to make it use regular work items instead of the special
> cpu->tcg_work_func, but that's pretty much it.

Ok, I think I have something.  It only uses condition variables when
there is a safe work in flight, to enter and leave the function at the
right time.  It also makes linux-user's start_exclusive/end_exclusive
use the same synchronization logic.  I'll test it and post; most
preliminary patches are straight from this series.

Paolo