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

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Message-ID: <c26e5b36-d369-4353-a5a8-9c9b381ce239@kernel.dk>
Date: Thu, 3 Oct 2024 08:38:48 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Dan Carpenter <dan.carpenter@...aro.org>
Cc: Waiman Long <longman@...hat.com>, Yu Kuai <yukuai3@...wei.com>,
 Tejun Heo <tj@...nel.org>, Josef Bacik <josef@...icpanda.com>,
 cgroups@...r.kernel.org, linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
 Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2] blk_iocost: remove some duplicate irq disable/enables

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 :-)

-- 
Jens Axboe