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SubjectRe: [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@kernel.org>

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

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

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