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

[Bart Van Assche <bart.vanassche@gmail.com>]

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.

Bart.