Re: [PATCH for-8.0 1/7] qemu/main-loop: Introduce QEMU_IOTHREAD_LOCK_GUARD

[Richard Henderson]

On 11/18/22 05:38, Alex Bennée wrote:
> Richard Henderson <richard.henderson@linaro.org> writes:
>
> > Create a wrapper for locking/unlocking the iothread lock.
> >
> > Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> > ---
> > Cc: Paolo Bonzini <pbonzini@redhat.com> (maintainer:Main loop)
> > ---
> >   include/qemu/main-loop.h | 29 +++++++++++++++++++++++++++++
> >   1 file changed, 29 insertions(+)
> >
> > diff --git a/include/qemu/main-loop.h b/include/qemu/main-loop.h
> > index 3c9a9a982d..c25f390696 100644
> > --- a/include/qemu/main-loop.h
> > +++ b/include/qemu/main-loop.h
> > @@ -343,6 +343,35 @@ void qemu_mutex_lock_iothread_impl(const char *file, int 
> > line);
> >    */
> >   void qemu_mutex_unlock_iothread(void);
> >   
> > +/**
> > + * QEMU_IOTHREAD_LOCK_GUARD
> > + *
> > + * Wrap a block of code in a conditional qemu_mutex_{lock,unlock}_iothread.
> > + */
> > +typedef struct IOThreadLockAuto IOThreadLockAuto;
> > +
> > +static inline IOThreadLockAuto *qemu_iothread_auto_lock(const char *file,
> > +                                                        int line)
> > +{
> > +    if (qemu_mutex_iothread_locked()) {
> > +        return NULL;
> > +    }
> > +    qemu_mutex_lock_iothread_impl(file, line);
> > +    /* Anything non-NULL causes the cleanup function to be called */
> > +    return (IOThreadLockAuto *)(uintptr_t)1;
>
> Oh hang on, what black magic is this. Does the compiler do a NULL check
> before calling the cleanup?

Not the compiler, but...

> > +G_DEFINE_AUTOPTR_CLEANUP_FUNC(IOThreadLockAuto, qemu_iothread_auto_unlock)

... this does.  Follow the macros down and you get

>   static G_GNUC_UNUSED inline void _GLIB_AUTOPTR_CLEAR_FUNC_NAME(TypeName) 
> (TypeName *_ptr)
>     { if (_ptr) (cleanup) ((ParentName *) _ptr); }                                                       

r~