linux-kernel - [PATCH 4/8] HMM: Remove superflous RCU protection around radix tree lookup

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Date:   Wed, 14 Mar 2018 12:45:11 -0700
From:   Tejun Heo <tj@...nel.org>
To:     torvalds@...ux-foundation.org, jannh@...gle.com,
        paulmck@...ux.vnet.ibm.com, bcrl@...ck.org,
        viro@...iv.linux.org.uk, kent.overstreet@...il.com
Cc:     security@...nel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, Tejun Heo <tj@...nel.org>, linux-mm@...ck.org
Subject: [PATCH 4/8] HMM: Remove superflous RCU protection around radix tree lookup

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, how do you want to route this patch?  If you prefer, I can
route it together with other patches.

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