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

From: Tejun Heo <tj@kernel.org>
To: torvalds@linux-foundation.org, jannh@google.com,
	paulmck@linux.vnet.ibm.com, bcrl@kvack.org,
	viro@zeniv.linux.org.uk, kent.overstreet@gmail.com
Cc: security@kernel.org, linux-kernel@vger.kernel.org,
	kernel-team@fb.com, "Tejun Heo" <tj@kernel.org>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	linux-mm@kvack.org
Subject: [PATCH 4/7] HMM: Remove superflous RCU protection around radix tree lookup
Date: Tue,  6 Mar 2018 09:33:13 -0800	[thread overview]
Message-ID: <20180306173316.3088458-4-tj@kernel.org> (raw)
In-Reply-To: <20180306173316.3088458-1-tj@kernel.org>

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: 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.

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
