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

[Paolo Bonzini]

On 02/08/2016 21:22, Emilio G. Cota wrote:
> An alternative is to have a separate lock for safe work, and check for
> safe work once there are no other locks held; a good place to do this is
> at the beginning of cpu_loop_exec. This scales better, and I'd argue
> it's simpler. In fact, I posted a patch that does this about a year
> ago (!):
>   https://lists.nongnu.org/archive/html/qemu-devel/2015-08/msg02576.html

I like the idea.

> Paolo didn't like condvars, but now I see them coming up again. I guess
> he still won't like the synchronize_rcu() call in there, and I don't like
> it either, but I don't think that's an essential part of that patch.

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.

Paolo