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

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

[bh1scw]

From: Fanjun Kong <bh1scw@gmail.com>

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@gmail.com>
---
 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);
-- 
2.36.0

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

[Tejun Heo]

On Tue, May 17, 2022 at 01:39:30AM +0800, bh1scw@gmail.com wrote:
> From: Fanjun Kong <bh1scw@gmail.com>
> 
> 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@gmail.com>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun

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

[Muchun Song]

On Tue, May 17, 2022 at 1:39 AM <bh1scw@gmail.com> wrote:
>
> From: Fanjun Kong <bh1scw@gmail.com>
>
> 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@gmail.com>

Reviewed-by: Muchun Song <songmuchun@bytedance.com>

Thanks.

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

[Jens Axboe]

On Tue, 17 May 2022 01:39:30 +0800, bh1scw@gmail.com wrote:
> From: Fanjun Kong <bh1scw@gmail.com>
> 
> 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().
> 
> 
> [...]

Applied, thanks!

[1/1] blk-cgroup: Remove unnecessary rcu_read_lock/unlock()
      commit: 77c570a1ea85ba4ab135c61a028420a6e9fe77f3

Best regards,
-- 
Jens Axboe

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

[Marek Szyprowski]

On 16.05.2022 19:39, bh1scw@gmail.com wrote:
> From: Fanjun Kong <bh1scw@gmail.com>
>
> 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@gmail.com>

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

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

[Jens Axboe]

On 5/18/22 1:28 PM, Marek Szyprowski wrote:
> On 16.05.2022 19:39, bh1scw@gmail.com wrote:
>> From: Fanjun Kong <bh1scw@gmail.com>
>>
>> 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@gmail.com>
> 
> 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

Should this use rcu_read_lock_any_held() rather than rcu_read_lock_held()?

-- 
Jens Axboe

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

[Jens Axboe]

On 5/18/22 4:29 PM, Jens Axboe wrote:
> On 5/18/22 1:28 PM, Marek Szyprowski wrote:
>> On 16.05.2022 19:39, bh1scw@gmail.com wrote:
>>> From: Fanjun Kong <bh1scw@gmail.com>
>>>
>>> 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@gmail.com>
>>
>> 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
> 
> Should this use rcu_read_lock_any_held() rather than
> rcu_read_lock_held()?

I think the better alternative is just to delete the WARN_ON(), we have
a:

lockdep_assert_held(&q->queue_lock);

right after it. Since the queue_lock is IRQ disabling, having two checks
serves no purpose. I'll kill the line.

-- 
Jens Axboe