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

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Date:   Tue, 6 Mar 2018 12:59:48 -0500
From:   Jerome Glisse <jglisse@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     torvalds@...ux-foundation.org, jannh@...gle.com,
        paulmck@...ux.vnet.ibm.com, bcrl@...ck.org,
        viro@...iv.linux.org.uk, kent.overstreet@...il.com,
        security@...nel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, linux-mm@...ck.org
Subject: Re: [PATCH 4/7] HMM: Remove superflous RCU protection around radix
 tree lookup

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@...nel.org>

Reviewed-by: Jérôme Glisse <jglisse@...hat.com>

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

> ---
> Hello, Jérôme.
> 
> 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
>