Re: [PATCH for-8.1] qemu-coroutine: remove qatomic_mb_read()

[Stefan Hajnoczi]

On Thu, 6 Apr 2023 at 06:09, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> Replace with an explicit barrier and a comment.
>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  util/qemu-coroutine.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/util/qemu-coroutine.c b/util/qemu-coroutine.c
> index 849452369201..17a88f65053e 100644
> --- a/util/qemu-coroutine.c
> +++ b/util/qemu-coroutine.c
> @@ -127,9 +127,13 @@ void qemu_aio_coroutine_enter(AioContext *ctx, Coroutine 
> *co)
>          Coroutine *to = QSIMPLEQ_FIRST(&pending);
>          CoroutineAction ret;
>
> -        /* Cannot rely on the read barrier for to in aio_co_wake(), as there 
> are
> -         * callers outside of aio_co_wake() */
> -        const char *scheduled = qatomic_mb_read(&to->scheduled);
> +        /*
> +         * Read to before to->scheduled; pairs with qatomic_cmpxchg in
> +         * qemu_co_sleep(), aio_co_schedule() etc.
> +         */
> +        smp_read_barrier_depends();

I'm not a fan of nuanced memory ordering primitives. I don't
understand or remember all the primitives available in
docs/devel/atomics.rst and especially not how they interact with each
other.

Does smp_read_barrier_depends() make sense for QEMU? Does QEMU support
Alpha host CPUs?

Stefan