[PATCH] cgroup, memcg, writeback: drop spurious rcu locking around mem_cgroup_css_from_page()

[PATCH] cgroup, memcg, writeback: drop spurious rcu locking around mem_cgroup_css_from_page()

[Tejun Heo]

In earlier versions, mem_cgroup_css_from_page() could return non-root
css on a legacy hierarchy which can go away and required rcu locking;
however, the eventual version simply returns the root cgroup if memcg
is on a legacy hierarchy and thus doesn't need rcu locking around or
in it.  Remove spurious rcu lockings.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Johannes Weiner <hannes@cmpxchg.org>
---
 fs/fs-writeback.c |    2 --
 mm/memcontrol.c   |    3 ---
 2 files changed, 5 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 023f6a1..6915c95 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -677,9 +677,7 @@ void wbc_account_io(struct writeback_control *wbc, struct page *page,
 	if (!wbc->wb)
 		return;
 
-	rcu_read_lock();
 	id = mem_cgroup_css_from_page(page)->id;
-	rcu_read_unlock();
 
 	if (id == wbc->wb_id) {
 		wbc->wb_bytes += bytes;
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index fc10620..3230201 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -431,14 +431,11 @@ struct cgroup_subsys_state *mem_cgroup_css_from_page(struct page *page)
 {
 	struct mem_cgroup *memcg;
 
-	rcu_read_lock();
-
 	memcg = page->mem_cgroup;
 
 	if (!memcg || !cgroup_subsys_on_dfl(memory_cgrp_subsys))
 		memcg = root_mem_cgroup;
 
-	rcu_read_unlock();
 	return &memcg->css;
 }
 

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