Re: [Qemu-devel] [PATCH v6 62/73] cpu: introduce cpu_has_work_with_iothread_lock

[Emilio G. Cota]

On Fri, Feb 08, 2019 at 11:33:32 +0000, Alex Bennée wrote:
> 
> Emilio G. Cota <address@hidden> writes:
> 
> > It will gain some users soon.
> >
> > Suggested-by: Paolo Bonzini <address@hidden>
> > Reviewed-by: Richard Henderson <address@hidden>
> > Signed-off-by: Emilio G. Cota <address@hidden>
> > ---
> >  include/qom/cpu.h | 36 +++++++++++++++++++++++++++++++++---
(snip)
> >  static inline bool cpu_has_work(CPUState *cpu)
> >  {
> >      CPUClass *cc = CPU_GET_CLASS(cpu);
> > +    bool has_cpu_lock = cpu_mutex_locked(cpu);
> > +    bool (*func)(CPUState *cpu);
> >      bool ret;
> >
> > +    if (cc->has_work_with_iothread_lock) {
> > +        if (qemu_mutex_iothread_locked()) {
> > +            func = cc->has_work_with_iothread_lock;
> > +            goto call_func;
> > +        }
> > +
> > +        if (has_cpu_lock) {
> > +            /* avoid deadlock by acquiring the locks in order */
> 
> This is fine here but can we expand the comment above:
> 
>  * cpu_has_work:
>  * @cpu: The vCPU to check.
>  *
>  * Checks whether the CPU has work to do. If the vCPU helper needs to
>  * check it's work status with the BQL held ensure we hold the BQL
>  * before taking the CPU lock.

I added a comment to the body of the function:

+    /* some targets require us to hold the BQL when checking for work */
     if (cc->has_work_with_iothread_lock) {

> Where is our canonical description of the locking interaction between
> BQL and CPU locks?

It's in a few places, for instance cpu_mutex_lock's documentation
in include/qom/cpu.h.

I've added a comment about the locking order to @lock's documentation,
also in cpu.h:

- * @lock: Lock to prevent multiple access to per-CPU fields.
+ * @lock: Lock to prevent multiple access to per-CPU fields. Must be acqrd
+ *        after the BQL.

> Otherwise:
> 
> Reviewed-by: Alex Bennée <address@hidden>

Thanks!

                Emilio