Re: [PATCH v2] blk_iocost: remove some duplicate irq disable/enables

[Waiman Long]

On 10/3/24 10:38, Jens Axboe wrote:
> On 10/3/24 8:31 AM, Dan Carpenter wrote:
> > On Thu, Oct 03, 2024 at 07:21:25AM -0600, Jens Axboe wrote:
> > > On 10/3/24 6:03 AM, Dan Carpenter wrote:
> > > >    3117                                  ioc_now(iocg->ioc, &now);
> > > >    3118                                  weight_updated(iocg, &now);
> > > >    3119                                  spin_unlock(&iocg->ioc->lock);
> > > >    3120                          }
> > > >    3121                  }
> > > >    3122                  spin_unlock_irq(&blkcg->lock);
> > > >    3123
> > > >    3124                  return nbytes;
> > > >    3125          }
> > > >    3126
> > > >    3127          blkg_conf_init(&ctx, buf);
> > > >    3128
> > > >    3129          ret = blkg_conf_prep(blkcg, &blkcg_policy_iocost, &ctx);
> > > >    3130          if (ret)
> > > >    3131                  goto err;
> > > >    3132
> > > >    3133          iocg = blkg_to_iocg(ctx.blkg);
> > > >    3134
> > > >    3135          if (!strncmp(ctx.body, "default", 7)) {
> > > >    3136                  v = 0;
> > > >    3137          } else {
> > > >    3138                  if (!sscanf(ctx.body, "%u", &v))
> > > >    3139                          goto einval;
> > > >    3140                  if (v < CGROUP_WEIGHT_MIN || v > CGROUP_WEIGHT_MAX)
> > > >    3141                          goto einval;
> > > >    3142          }
> > > >    3143
> > > >    3144          spin_lock(&iocg->ioc->lock);
> > > >
> > > > But why is this not spin_lock_irq()?  I haven't analyzed this so maybe it's
> > > > fine.
> > > That's a bug.
> > I could obviously write this patch but I feel stupid writing the
> > commit message. My level of understanding is Monkey See Monkey do.
> > Could you take care of this?
> Sure - or let's add Tejun who knows this code better. Ah he's already
> added. Tejun?
>
> > So somewhere we're taking a lock in the IRQ handler and this can lead
> > to a deadlock? I thought this would have been caught by lockdep?
> It's nested inside blkcg->lock which is IRQ safe, that is enough. But
> doing a quick scan of the file, the usage is definitely (widly)
> inconsistent. Most times ioc->lock is grabbed disabling interrupts, but
> there are also uses that doesn't disable interrupts, coming from things
> like seq_file show paths which certainly look like they need it. lockdep
> should certainly warn about this, only explanation I have is that nobody
> bothered to do that :-)

The lockdep validator will only warn about this if a debug kernel with 
lockdep enabled has run a workload that exercises all the relevant 
locking sequences that can implicate a potential for deadlock.

Cheers,
Longman