Re: [RFC PATCH 2/6] job: _locked functions and public job_lock/unlock for next patch

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Jul 12, 2021 at 10:43:07AM +0200, Emanuele Giuseppe Esposito wrote:
> 
> 
> On 08/07/2021 12:50, Stefan Hajnoczi wrote:
> > On Wed, Jul 07, 2021 at 06:58:09PM +0200, Emanuele Giuseppe Esposito wrote:
> > > diff --git a/job.c b/job.c
> > > index 872bbebb01..96fb8e9730 100644
> > > --- a/job.c
> > > +++ b/job.c
> > > @@ -32,6 +32,10 @@
> > >   #include "trace/trace-root.h"
> > >   #include "qapi/qapi-events-job.h"
> > > +/* job_mutex protexts the jobs list, but also the job operations. */
> > > +static QemuMutex job_mutex;
> > 
> > It's unclear what protecting "job operations" means. I would prefer a
> > fine-grained per-job lock that protects the job's fields instead of a
> > global lock with an unclear scope.
> 
> As I wrote in the cover letter, I wanted to try to keep things as simple as
> possible with a global lock. It is possible to try and have a per-job lock,
> but I don't know how complex will that be then.
> I will try and see what I can do.
> 
> Maybe "job_mutex protexts the jobs list, but also makes the job API
> thread-safe"?

That's clearer, thanks. I thought "job operations" meant the processing
that the actual block jobs do (commit, mirror, stream, backup).

> 
> > 
> > > +
> > > +/* Protected by job_mutex */
> > >   static QLIST_HEAD(, Job) jobs = QLIST_HEAD_INITIALIZER(jobs);
> > >   /* Job State Transition Table */
> > > @@ -64,27 +68,22 @@ bool JobVerbTable[JOB_VERB__MAX][JOB_STATUS__MAX] = {
> > >   /* Transactional group of jobs */
> > >   struct JobTxn {
> > > -    /* Is this txn being cancelled? */
> > > +    /* Is this txn being cancelled? Atomic.*/
> > >       bool aborting;
> > 
> > The comment says atomic but this field is not accessed using atomic
> > operations (at least at this point in the patch series)?
> 
> Yes sorry I messed up the hunks in one-two patches. These comments were
> supposed to be on patch 4 "job.h: categorize job fields". Even though that
> might also not be ideal, since that patch just introduces the comments,
> without applying the locking/protection yet.
> On the other side, if I merge everything together in patch 5, it will be
> even harder to read.

The commit description can describe changes that currently have no
effect but are anticipating a later patch. That helps reviewers
understand whether the change is intentional/correct.

Stefan

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