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
