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[PATCH] block, bfq: switch 'bfqg->ref' to use atomic refcount apis

Yu Kuai posted 1 patch 2 years, 8 months ago 
block/bfq-cgroup.c  | 8 +++-----
block/bfq-iosched.h | 2 +-
2 files changed, 4 insertions(+), 6 deletions(-)
[PATCH] block, bfq: switch 'bfqg->ref' to use atomic refcount apis
Posted by Yu Kuai 2 years, 8 months ago 
From: Yu Kuai <yukuai3@huawei.com>

The updating of 'bfqg->ref' should be protected by 'bfqd->lock', however,
during code review, we found that bfq_pd_free() update 'bfqg->ref'
without holding the lock, which is problematic:

1) bfq_pd_free() triggered by removing cgroup is called asynchronously;
2) bfqq will grab bfqg reference, and exit bfqq will drop the reference,
which can concurrent with 1).

Unfortunately, 'bfqd->lock' can't be held here because 'bfqd' might already
be freed in bfq_pd_free(). Fix the problem by using atomic refcount apis.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
---
Changes from RFC:
 - refcount_dec_and_test(bfqg->ref) -> refcount_dec_and_test(&bfqg->ref)

 block/bfq-cgroup.c  | 8 +++-----
 block/bfq-iosched.h | 2 +-
 2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
index 1b2829e99dad..7d9b15f0dbd5 100644
--- a/block/bfq-cgroup.c
+++ b/block/bfq-cgroup.c
@@ -316,14 +316,12 @@ struct bfq_group *bfqq_group(struct bfq_queue *bfqq)
 
 static void bfqg_get(struct bfq_group *bfqg)
 {
-	bfqg->ref++;
+	refcount_inc(&bfqg->ref);
 }
 
 static void bfqg_put(struct bfq_group *bfqg)
 {
-	bfqg->ref--;
-
-	if (bfqg->ref == 0)
+	if (refcount_dec_and_test(&bfqg->ref))
 		kfree(bfqg);
 }
 
@@ -530,7 +528,7 @@ static struct blkg_policy_data *bfq_pd_alloc(gfp_t gfp, struct request_queue *q,
 	}
 
 	/* see comments in bfq_bic_update_cgroup for why refcounting */
-	bfqg_get(bfqg);
+	refcount_set(&bfqg->ref, 1);
 	return &bfqg->pd;
 }
 
diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
index 41aa151ccc22..466e4865ace6 100644
--- a/block/bfq-iosched.h
+++ b/block/bfq-iosched.h
@@ -928,7 +928,7 @@ struct bfq_group {
 	char blkg_path[128];
 
 	/* reference counter (see comments in bfq_bic_update_cgroup) */
-	int ref;
+	refcount_t ref;
 	/* Is bfq_group still online? */
 	bool online;
 
-- 
2.31.1
Re: [PATCH] block, bfq: switch 'bfqg->ref' to use atomic refcount apis
Posted by Jens Axboe 2 years, 8 months ago 
On Tue, 03 Jan 2023 16:47:55 +0800, Yu Kuai wrote:
> The updating of 'bfqg->ref' should be protected by 'bfqd->lock', however,
> during code review, we found that bfq_pd_free() update 'bfqg->ref'
> without holding the lock, which is problematic:
> 
> 1) bfq_pd_free() triggered by removing cgroup is called asynchronously;
> 2) bfqq will grab bfqg reference, and exit bfqq will drop the reference,
> which can concurrent with 1).
> 
> [...]

Applied, thanks!

[1/1] block, bfq: switch 'bfqg->ref' to use atomic refcount apis
      commit: 216f764716f34fe68cedc7296ae2043a7727e640

Best regards,
-- 
Jens Axboe
Re: [PATCH] block, bfq: switch 'bfqg->ref' to use atomic refcount apis
Posted by Yu Kuai 2 years, 8 months ago 
Hi, Jens

在 2023/01/03 16:47, Yu Kuai 写道:
> From: Yu Kuai <yukuai3@huawei.com>
> 
> The updating of 'bfqg->ref' should be protected by 'bfqd->lock', however,
> during code review, we found that bfq_pd_free() update 'bfqg->ref'
> without holding the lock, which is problematic:
> 
> 1) bfq_pd_free() triggered by removing cgroup is called asynchronously;
> 2) bfqq will grab bfqg reference, and exit bfqq will drop the reference,
> which can concurrent with 1).
> 
> Unfortunately, 'bfqd->lock' can't be held here because 'bfqd' might already
> be freed in bfq_pd_free(). Fix the problem by using atomic refcount apis.
> 
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>

Can you apply this patch?

Thanks,
Kuai
> ---
> Changes from RFC:
>   - refcount_dec_and_test(bfqg->ref) -> refcount_dec_and_test(&bfqg->ref)
> 
>   block/bfq-cgroup.c  | 8 +++-----
>   block/bfq-iosched.h | 2 +-
>   2 files changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
> index 1b2829e99dad..7d9b15f0dbd5 100644
> --- a/block/bfq-cgroup.c
> +++ b/block/bfq-cgroup.c
> @@ -316,14 +316,12 @@ struct bfq_group *bfqq_group(struct bfq_queue *bfqq)
>   
>   static void bfqg_get(struct bfq_group *bfqg)
>   {
> -	bfqg->ref++;
> +	refcount_inc(&bfqg->ref);
>   }
>   
>   static void bfqg_put(struct bfq_group *bfqg)
>   {
> -	bfqg->ref--;
> -
> -	if (bfqg->ref == 0)
> +	if (refcount_dec_and_test(&bfqg->ref))
>   		kfree(bfqg);
>   }
>   
> @@ -530,7 +528,7 @@ static struct blkg_policy_data *bfq_pd_alloc(gfp_t gfp, struct request_queue *q,
>   	}
>   
>   	/* see comments in bfq_bic_update_cgroup for why refcounting */
> -	bfqg_get(bfqg);
> +	refcount_set(&bfqg->ref, 1);
>   	return &bfqg->pd;
>   }
>   
> diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
> index 41aa151ccc22..466e4865ace6 100644
> --- a/block/bfq-iosched.h
> +++ b/block/bfq-iosched.h
> @@ -928,7 +928,7 @@ struct bfq_group {
>   	char blkg_path[128];
>   
>   	/* reference counter (see comments in bfq_bic_update_cgroup) */
> -	int ref;
> +	refcount_t ref;
>   	/* Is bfq_group still online? */
>   	bool online;
>   
>
Re: [PATCH] block, bfq: switch 'bfqg->ref' to use atomic refcount apis
Posted by Jan Kara 2 years, 8 months ago 
On Tue 03-01-23 16:47:55, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.com>
> 
> The updating of 'bfqg->ref' should be protected by 'bfqd->lock', however,
> during code review, we found that bfq_pd_free() update 'bfqg->ref'
> without holding the lock, which is problematic:
> 
> 1) bfq_pd_free() triggered by removing cgroup is called asynchronously;
> 2) bfqq will grab bfqg reference, and exit bfqq will drop the reference,
> which can concurrent with 1).
> 
> Unfortunately, 'bfqd->lock' can't be held here because 'bfqd' might already
> be freed in bfq_pd_free(). Fix the problem by using atomic refcount apis.
> 
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>

The patch looks good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
> Changes from RFC:
>  - refcount_dec_and_test(bfqg->ref) -> refcount_dec_and_test(&bfqg->ref)
> 
>  block/bfq-cgroup.c  | 8 +++-----
>  block/bfq-iosched.h | 2 +-
>  2 files changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
> index 1b2829e99dad..7d9b15f0dbd5 100644
> --- a/block/bfq-cgroup.c
> +++ b/block/bfq-cgroup.c
> @@ -316,14 +316,12 @@ struct bfq_group *bfqq_group(struct bfq_queue *bfqq)
>  
>  static void bfqg_get(struct bfq_group *bfqg)
>  {
> -	bfqg->ref++;
> +	refcount_inc(&bfqg->ref);
>  }
>  
>  static void bfqg_put(struct bfq_group *bfqg)
>  {
> -	bfqg->ref--;
> -
> -	if (bfqg->ref == 0)
> +	if (refcount_dec_and_test(&bfqg->ref))
>  		kfree(bfqg);
>  }
>  
> @@ -530,7 +528,7 @@ static struct blkg_policy_data *bfq_pd_alloc(gfp_t gfp, struct request_queue *q,
>  	}
>  
>  	/* see comments in bfq_bic_update_cgroup for why refcounting */
> -	bfqg_get(bfqg);
> +	refcount_set(&bfqg->ref, 1);
>  	return &bfqg->pd;
>  }
>  
> diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
> index 41aa151ccc22..466e4865ace6 100644
> --- a/block/bfq-iosched.h
> +++ b/block/bfq-iosched.h
> @@ -928,7 +928,7 @@ struct bfq_group {
>  	char blkg_path[128];
>  
>  	/* reference counter (see comments in bfq_bic_update_cgroup) */
> -	int ref;
> +	refcount_t ref;
>  	/* Is bfq_group still online? */
>  	bool online;
>  
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Patchew 2.1-devel-b67e4c2

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