Re: [PATCH 5/6] hmp: convert handle_hmp_command() to AIO_WAIT_WHILE_UNLOCKED()

[Markus Armbruster <armbru@redhat.com>]

Stefan Hajnoczi <stefanha@redhat.com> writes:

> The HMP monitor runs in the main loop thread. Calling

Correct.

> AIO_WAIT_WHILE(qemu_get_aio_context(), ...) from the main loop thread is
> equivalent to AIO_WAIT_WHILE_UNLOCKED(NULL, ...) because neither unlocks
> the AioContext and the latter's assertion that we're in the main loop
> succeeds.
>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  monitor/hmp.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/monitor/hmp.c b/monitor/hmp.c
> index 2aa85d3982..5ecbdac802 100644
> --- a/monitor/hmp.c
> +++ b/monitor/hmp.c
> @@ -1167,7 +1167,7 @@ void handle_hmp_command(MonitorHMP *mon, const char *cmdline)
>          Coroutine *co = qemu_coroutine_create(handle_hmp_command_co, &data);
>          monitor_set_cur(co, &mon->common);
>          aio_co_enter(qemu_get_aio_context(), co);
> -        AIO_WAIT_WHILE(qemu_get_aio_context(), !data.done);
> +        AIO_WAIT_WHILE_UNLOCKED(NULL, !data.done);
>      }
>  
>      qobject_unref(qdict);

Acked-by: Markus Armbruster <armbru@redhat.com>

For an R-by, I need to understand this in more detail.  I'm not familiar
with the innards of AIO_WAIT_WHILE() & friends, so I need to go real
slow.

We change

    ctx from qemu_get_aio_context() to NULL
    unlock from true to false

in

    bool waited_ = false;                                          \
    AioWait *wait_ = &global_aio_wait;                             \
    AioContext *ctx_ = (ctx);                                      \
    /* Increment wait_->num_waiters before evaluating cond. */     \
    qatomic_inc(&wait_->num_waiters);                              \
    /* Paired with smp_mb in aio_wait_kick(). */                   \
    smp_mb();                                                      \
    if (ctx_ && in_aio_context_home_thread(ctx_)) {                \
        while ((cond)) {                                           \
            aio_poll(ctx_, true);                                  \
            waited_ = true;                                        \
        }                                                          \
    } else {                                                       \
        assert(qemu_get_current_aio_context() ==                   \
               qemu_get_aio_context());                            \
        while ((cond)) {                                           \
            if (unlock && ctx_) {                                  \
                aio_context_release(ctx_);                         \
            }                                                      \
            aio_poll(qemu_get_aio_context(), true);                \
            if (unlock && ctx_) {                                  \
                aio_context_acquire(ctx_);                         \
            }                                                      \
            waited_ = true;                                        \
        }                                                          \
    }                                                              \
    qatomic_dec(&wait_->num_waiters);                              \
    waited_; })

qemu_get_aio_context() is non-null here, correct?

What's the value of in_aio_context_home_thread(qemu_get_aio_context())?