[PATCH 4/7] HMM: Remove superflous RCU protection around radix tree lookup

[PATCH 4/7] HMM: Remove superflous RCU protection around radix tree lookup

[Tejun Heo]

hmm_devmem_find() requires rcu_read_lock_held() but there's nothing
which actually uses the RCU protection.  The only caller is
hmm_devmem_pages_create() which already grabs the mutex and does
superflous rcu_read_lock/unlock() around the function.

This doesn't add anything and just adds to confusion.  Remove the RCU
protection and open-code the radix tree lookup.  If this needs to
become more sophisticated in the future, let's add them back when
necessary.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: JA(C)rA'me Glisse <jglisse@redhat.com>
Cc: linux-mm@kvack.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>
---
Hello, JA(C)rA'me.

This came up while auditing percpu_ref users for missing explicit RCU
grace periods.  HMM doesn't seem to depend on RCU protection at all,
so I thought it'd be better to remove it for now.  It's only compile
tested.

Thanks.

 mm/hmm.c | 12 ++----------
 1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index 320545b98..d4627c5 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -845,13 +845,6 @@ static void hmm_devmem_release(struct device *dev, void *data)
 	hmm_devmem_radix_release(resource);
 }
 
-static struct hmm_devmem *hmm_devmem_find(resource_size_t phys)
-{
-	WARN_ON_ONCE(!rcu_read_lock_held());
-
-	return radix_tree_lookup(&hmm_devmem_radix, phys >> PA_SECTION_SHIFT);
-}
-
 static int hmm_devmem_pages_create(struct hmm_devmem *devmem)
 {
 	resource_size_t key, align_start, align_size, align_end;
@@ -892,9 +885,8 @@ static int hmm_devmem_pages_create(struct hmm_devmem *devmem)
 	for (key = align_start; key <= align_end; key += PA_SECTION_SIZE) {
 		struct hmm_devmem *dup;
 
-		rcu_read_lock();
-		dup = hmm_devmem_find(key);
-		rcu_read_unlock();
+		dup = radix_tree_lookup(&hmm_devmem_radix,
+					key >> PA_SECTION_SHIFT);
 		if (dup) {
 			dev_err(device, "%s: collides with mapping for %s\n",
 				__func__, dev_name(dup->device));
-- 
2.9.5

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Re: [PATCH 4/7] HMM: Remove superflous RCU protection around radix tree lookup

[Jerome Glisse]

On Tue, Mar 06, 2018 at 09:33:13AM -0800, Tejun Heo wrote:
> hmm_devmem_find() requires rcu_read_lock_held() but there's nothing
> which actually uses the RCU protection.  The only caller is
> hmm_devmem_pages_create() which already grabs the mutex and does
> superflous rcu_read_lock/unlock() around the function.
> 
> This doesn't add anything and just adds to confusion.  Remove the RCU
> protection and open-code the radix tree lookup.  If this needs to
> become more sophisticated in the future, let's add them back when
> necessary.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>

Reviewed-by: Jerome Glisse <jglisse@redhat.com>

> Cc: linux-mm@kvack.org
> Cc: Linus Torvalds <torvalds@linux-foundation.org>

> ---
> Hello, Jerome.
> 
> This came up while auditing percpu_ref users for missing explicit RCU
> grace periods.  HMM doesn't seem to depend on RCU protection at all,
> so I thought it'd be better to remove it for now.  It's only compile
> tested.

Good catch some left over of old logic. I have more cleanup queued up
now that i am about to post nouveau patches to use all this. Thanks for
fixing this.

> 
> Thanks.
> 
>  mm/hmm.c | 12 ++----------
>  1 file changed, 2 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/hmm.c b/mm/hmm.c
> index 320545b98..d4627c5 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -845,13 +845,6 @@ static void hmm_devmem_release(struct device *dev, void *data)
>  	hmm_devmem_radix_release(resource);
>  }
>  
> -static struct hmm_devmem *hmm_devmem_find(resource_size_t phys)
> -{
> -	WARN_ON_ONCE(!rcu_read_lock_held());
> -
> -	return radix_tree_lookup(&hmm_devmem_radix, phys >> PA_SECTION_SHIFT);
> -}
> -
>  static int hmm_devmem_pages_create(struct hmm_devmem *devmem)
>  {
>  	resource_size_t key, align_start, align_size, align_end;
> @@ -892,9 +885,8 @@ static int hmm_devmem_pages_create(struct hmm_devmem *devmem)
>  	for (key = align_start; key <= align_end; key += PA_SECTION_SIZE) {
>  		struct hmm_devmem *dup;
>  
> -		rcu_read_lock();
> -		dup = hmm_devmem_find(key);
> -		rcu_read_unlock();
> +		dup = radix_tree_lookup(&hmm_devmem_radix,
> +					key >> PA_SECTION_SHIFT);
>  		if (dup) {
>  			dev_err(device, "%s: collides with mapping for %s\n",
>  				__func__, dev_name(dup->device));
> -- 
> 2.9.5
> 

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