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

[Waiman Long <longman@redhat.com>]

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