Re: [PATCH] blk-iocost: use irq-safe locking in cgroup handlers

[Yu Kuai]

Hi,

在 2026/6/2 21:25, Jens Axboe 写道:
> On 6/1/26 3:50 PM, Bart Van Assche wrote:
>> On 5/31/26 11:13 PM, Yu Kuai wrote:
>>> @@ -3378,14 +3378,14 @@ static u64 ioc_cost_model_prfill(struct seq_file *sf,
>>>        if (!dname)
>>>            return 0;
>>>    -    spin_lock(&ioc->lock);
>>> +    spin_lock_irq(&ioc->lock);
>>>        seq_printf(sf, "%s ctrl=%s model=linear "
>>>               "rbps=%llu rseqiops=%llu rrandiops=%llu "
>>>               "wbps=%llu wseqiops=%llu wrandiops=%llu\n",
>>>               dname, ioc->user_cost_model ? "user" : "auto",
>>>               u[I_LCOEF_RBPS], u[I_LCOEF_RSEQIOPS], u[I_LCOEF_RRANDIOPS],
>>>               u[I_LCOEF_WBPS], u[I_LCOEF_WSEQIOPS], u[I_LCOEF_WRANDIOPS]);
>>> -    spin_unlock(&ioc->lock);
>>> +    spin_unlock_irq(&ioc->lock);
>>>        return 0;
>>>    }
>> This change is wrong. ioc_cost_model_prfill() only has one caller,
>> namely blkcg_print_blkgs(). blkcg_print_blkgs() calls the above function
>> with interrupts disabled. The spin_unlock_irq(&ioc->lock) at the end of
>> the above function enables interrupts while q->queue_lock is held. If an
>> interrupt happens on the same CPU core before q->queue_lock is unlocked,
>> and that interrupt tries to lock q->queue_lock, a deadlock will occur.
> Agree, it's broken. Which makes me suspect of the traces shown. Yu,
> can you please shed some light on this?

Looks like my reply is in your spam again :(

The trace is from ioc_weight_write(), which do have the problem. And
while reviewing related code, I'm wrong to think ioc_cost_model_prfill()
have the same problem and changed it as well.

>
> I've dropped it, thanks Bart.

I'll send a v2, and only fix ioc_weight_write().

>
-- 
Thansk,
Kuai