[PATCH] writeback: fix obtain a reference to a freeing memcg css

[Muchun Song]

The caller of wb_get_create() should pin the memcg, because
wb_get_create() relies on this guarantee. The rcu read lock
only can guarantee that the memcg css returned by css_from_id()
cannot be released, but the reference of the memcg can be zero.
Fix it by holding a reference to the css before calling
wb_get_create(). This is not a problem I encountered in the
real world. Just the result of a code review.

Fixes: 682aa8e1a6a1 ("writeback: implement unlocked_inode_to_wb transaction and use it for stat updates")
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 fs/fs-writeback.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 3ac002561327..afa658ffc09f 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -506,8 +506,10 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
 	/* find and pin the new wb */
 	rcu_read_lock();
 	memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
-	if (memcg_css)
+	if (memcg_css && css_tryget(memcg_css)) {
 		isw->new_wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
+		css_put(memcg_css);
+	}
 	rcu_read_unlock();
 	if (!isw->new_wb)
 		goto out_free;
-- 
2.11.0