On Thu, Mar 02, 2023 at 04:02:22PM +0100, Markus Armbruster wrote:
> Stefan Hajnoczi <stefa...@redhat.com> writes:
>
> > On Thu, Mar 02, 2023 at 08:17:43AM +0100, Markus Armbruster wrote:
> >> Stefan Hajnoczi <stefa...@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 <stefa...@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 <arm...@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?
> >
> > qemu_get_aio_context() always returns the main loop thread's AioContext.
>
> So it's non-null.
Yes. Sorry, I should have answered directly :).
> > qemu_get_current_aio_context() returns the AioContext that was most
> > recently set in the my_aiocontext thread-local variable for IOThreads,
> > the main loop's AioContext for BQL threads, or NULL for threads
> > that don't use AioContext at all.
> >
> >> What's the value of in_aio_context_home_thread(qemu_get_aio_context())?
> >
> > This function checks whether the given AioContext is associated with
> > this thread. In a BQL thread it returns true if the context is the main
> > loop's AioContext. In an IOThread it returns true if the context is the
> > IOThread's AioContext. Otherwise it returns false.
>
> I guess that means in_aio_context_home_thread(qemu_get_aio_context()) is
> true in the main thread.
>
> Before the patch, the if's condition is true, and we execute
>
> while ((cond)) { \
> aio_poll(ctx_, true); \
> waited_ = true; \
> } \
>
> Afterwards, it's false, and we execute
>
> >> } \
> >> qatomic_dec(&wait_->num_waiters); \
> >> waited_; })
> >>
> >> qemu_get_aio_context() is non-null here, correct?
> >
> > qemu_get_aio_context() always returns the main loop thread's AioContext.
>
> So it's non-null.
>
> > qemu_get_current_aio_context() returns the AioContext that was most
> > recently set in the my_aiocontext thread-local variable for IOThreads,
> > the main loop's AioContext for BQL threads, or NULL for threads
> > that don't use AioContext at all.
> >
> >> What's the value of in_aio_context_home_thread(qemu_get_aio_context())?
> >
> > This function checks whether the given AioContext is associated with
> > this thread. In a BQL thread it returns true if the context is the main
> > loop's AioContext. In an IOThread it returns true if the context is the
> > IOThread's AioContext. Otherwise it returns false.
>
> I guess that means in_aio_context_home_thread(qemu_get_aio_context()) is
> true in the main thread.
Yes.
> Before the patch, the if's condition is true, and we execute
>
> while ((cond)) { \
> aio_poll(ctx_, true); \
> waited_ = true; \
> } \
>
> Afterwards, it's false, and we instead execute
>
> 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; \
> } \
>
> The assertion is true: both operands of == are the main loop's
> AioContext.
Yes.
> The if conditions are false, because unlock is.
>
> Therefore, we execute the exact same code.
>
> All correct?
Yes, exactly.
Stefan
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