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

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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





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