Re: [PATCH] system/runstate: Fix regression, clarify BQL status of exit notifiers

[Paolo Bonzini <pbonzini@redhat.com>]

On 1/12/25 22:26, Phil Dennis-Jordan wrote:
> By changing the way the main QEMU event loop is invoked, I inadvertently
> changed the BQL status of exit notifiers: some of them implicitly
> assumed they would be called with the BQL held; the BQL is however
> not held during the exit(status) call in qemu_default_main().
> 
> Instead of attempting to ensuring we always call exit() from the BQL -
> including any transitive calls - this change adds a BQL lock guard to
> qemu_run_exit_notifiers, ensuring the BQL will always be held in the
> exit notifiers.
> 
> Additionally, the BQL promise is now documented at the
> qemu_{add,remove}_exit_notifier() declarations.
> 
> Fixes: f5ab12caba4f ("ui & main loop: Redesign of system-specific main
> thread event handling")
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2771
> Reported-by: David Woodhouse <dwmw2@infradead.org>
> Signed-off-by: Phil Dennis-Jordan <phil@philjordan.eu>

I'm worried that this breaks for exit() calls that happen within a 
BQL-taken area (for example, anything that uses error_fatal) due to...

void bql_lock_impl(const char *file, int line)
{
     QemuMutexLockFunc bql_lock_fn = qatomic_read(&bql_mutex_lock_func);

     g_assert(!bql_locked()); // <--- this
     bql_lock_fn(&bql, file, line);
     set_bql_locked(true);
}

Paolo