Re: [RFC PATCH v2 24/25] job.h: split function pointers in JobDriver

[Stefan Hajnoczi]

On Fri, Oct 08, 2021 at 12:48:26PM +0200, Emanuele Giuseppe Esposito wrote:
> 
> 
> On 07/10/2021 16:54, Stefan Hajnoczi wrote:
> > On Tue, Oct 05, 2021 at 10:32:14AM -0400, Emanuele Giuseppe Esposito wrote:
> > > The job API will be handled separately in another serie.
> > > 
> > > Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
> > > ---
> > >   include/qemu/job.h | 31 +++++++++++++++++++++++++++++++
> > >   1 file changed, 31 insertions(+)
> > > 
> > > diff --git a/include/qemu/job.h b/include/qemu/job.h
> > > index 41162ed494..c236c43026 100644
> > > --- a/include/qemu/job.h
> > > +++ b/include/qemu/job.h
> > > @@ -169,12 +169,21 @@ typedef struct Job {
> > >    * Callbacks and other information about a Job driver.
> > >    */
> > >   struct JobDriver {
> > > +
> > > +    /* Fields initialized in struct definition and never changed. */
> > > +
> > >       /** Derived Job struct size */
> > >       size_t instance_size;
> > >       /** Enum describing the operation */
> > >       JobType job_type;
> > > +    /*
> > > +     * I/O API functions. These functions are thread-safe, and therefore
> > > +     * can run in any thread as long as they have called
> > > +     * aio_context_acquire/release().
> > > +     */
> > 
> > This comment described the block layer well but job.h is more generic. I
> > don't think these functions are necessarily thread-safe (they shouldn't
> > be invoked from multiple threads at the same time). It's just that they
> > are run without regard to the BQL and may run in an arbitrary thread.
> 
> Ok, in this instance I will change it to:
> 
> /*
>  * Functions run without regard to the BQL and may run in any
>  * arbitrary thread.
>  */

It's not clear from that comment whether the functions need to be
thread-safe (re-entrancy safe) or not. I think the answer is "no" since
the caller ensures they are called from one thread at a time.

Stefan

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