Re: [PATCH] blk-cgroup: Remove unnecessary rcu_read_lock/unlock()

[Marek Szyprowski]

On 16.05.2022 19:39, bh1scw@xxxxxxxxx wrote:
> From: Fanjun Kong <bh1scw@xxxxxxxxx>
>
> spin_lock_irq/spin_unlock_irq contains preempt_disable/enable().
> Which can serve as RCU read-side critical region, so remove
> rcu_read_lock/unlock().
>
> Signed-off-by: Fanjun Kong <bh1scw@xxxxxxxxx>

This patch landed in today's linux next-20220518 as commit 77c570a1ea85 
("blk-cgroup: Remove unnecessary rcu_read_lock/unlock()").

Unfortunately it triggers the following warning on ARM64 based Raspberry 
Pi 4B board:

------------[ cut here ]------------
WARNING: CPU: 0 PID: 1 at block/blk-cgroup.c:301 blkg_create+0x398/0x4e0
Modules linked in:
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.18.0-rc3+ #5080
Hardware name: Raspberry Pi 4 Model B (DT)
pstate: 600000c5 (nZCv daIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : blkg_create+0x398/0x4e0
...
Call trace:
  blkg_create+0x398/0x4e0
  blkcg_init_queue+0x74/0x204
  __alloc_disk_node+0xf8/0x1f0
  __blk_alloc_disk+0x38/0x140
  brd_alloc.part.0+0xf8/0x220
  brd_init+0xe8/0x164
  do_one_initcall+0x74/0x400
  kernel_init_freeable+0x2f4/0x37c
  kernel_init+0x28/0x130
  ret_from_fork+0x10/0x20
irq event stamp: 218372
hardirqs last  enabled at (218371): [<ffff80000914b99c>] 
_raw_spin_unlock_irqrestore+0x98/0x9c
hardirqs last disabled at (218372): [<ffff80000914bcbc>] 
_raw_spin_lock_irq+0xac/0xb0
softirqs last  enabled at (216732): [<ffff800008010470>] _stext+0x470/0x5e8
softirqs last disabled at (216723): [<ffff8000080a0ec4>] 
__irq_exit_rcu+0x180/0x1ac
---[ end trace 0000000000000000 ]---

If this is a false positive, then the check in the code needs to be 
adjusted.

> ---
>   block/blk-cgroup.c | 4 +---
>   1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> index a91f8ae18b49..7bdc16a36560 100644
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -1283,14 +1283,13 @@ int blkcg_init_queue(struct request_queue *q)
>   	preloaded = !radix_tree_preload(GFP_KERNEL);
>   
>   	/* Make sure the root blkg exists. */
> -	rcu_read_lock();
> +	/* spin_lock_irq can serve as RCU read-side critical section. */
>   	spin_lock_irq(&q->queue_lock);
>   	blkg = blkg_create(&blkcg_root, q, new_blkg);
>   	if (IS_ERR(blkg))
>   		goto err_unlock;
>   	q->root_blkg = blkg;
>   	spin_unlock_irq(&q->queue_lock);
> -	rcu_read_unlock();
>   
>   	if (preloaded)
>   		radix_tree_preload_end();
> @@ -1316,7 +1315,6 @@ int blkcg_init_queue(struct request_queue *q)
>   	return ret;
>   err_unlock:
>   	spin_unlock_irq(&q->queue_lock);
> -	rcu_read_unlock();
>   	if (preloaded)
>   		radix_tree_preload_end();
>   	return PTR_ERR(blkg);

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland