Re: [Qemu-devel] [RFC PATCH v4 13/23] cpus: only take BQL for sleeping threads

["Pavel Dovgalyuk" <dovgaluk@ispras.ru>]

> -----Original Message-----
> From: Paolo Bonzini [mailto:pbonzini@redhat.com]
> Sent: Friday, January 19, 2018 3:20 PM
> To: Pavel Dovgalyuk; 'Pavel Dovgalyuk'; qemu-devel@nongnu.org
> Cc: kwolf@redhat.com; peter.maydell@linaro.org; boost.lists@gmail.com; quintela@redhat.com;
> jasowang@redhat.com; mst@redhat.com; zuban32s@gmail.com; maria.klimushenkova@ispras.ru;
> kraxel@redhat.com; alex.bennee@linaro.org
> Subject: Re: [RFC PATCH v4 13/23] cpus: only take BQL for sleeping threads
> 
> On 19/01/2018 13:05, Pavel Dovgalyuk wrote:
> >> From: Paolo Bonzini [mailto:pbonzini@redhat.com]
> >> On 19/01/2018 09:44, Pavel Dovgalyuk wrote:
> >>>      while (all_cpu_threads_idle()) {
> >>> +        qemu_mutex_lock_iothread();
> >>>          stop_tcg_kick_timer();
> >>>          qemu_cond_wait(cpu->halt_cond, &qemu_global_mutex);
> >>> +        qemu_mutex_unlock_iothread();
> >>>      }
> >>
> >> cpu_has_work cannot be called outside BQL yet.  You first need to access
> >> cpu->interrupt_request with atomics.
> >>
> >> In general, testing the condition outside the mutex is a very dangerous
> >> pattern (and I'm usually the one who enjoys dangerous patterns).
> >
> > It means, that I'll have to fix all the has_work function to avoid races,
> > because x86_cpu_has_work may have them?
> 
> Why only x86_cpu_has_work?
> 
> Even reading cs->interrupt_request outside the mutex is unsafe.

All the vcpu function that access interrupt controller or peripheral state may be unsafe?
How can it work safely then?

Pavel Dovgalyuk