[PATCH 4.19 161/421] writeback: fix obtain a reference to a freeing memcg css

From: Greg Kroah-Hartman
Date: Mon Jul 19 2021 - 12:17:32 EST

From: Muchun Song <songmuchun@xxxxxxxxxxxxx>

[ Upstream commit 8b0ed8443ae6458786580d36b7d5f8125535c5d4 ]

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.

memcg_css = css_from_id()
// css_get can change the ref counter from 0 back to 1

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")
Link: https://lore.kernel.org/r/20210402091145.80635-1-songmuchun@xxxxxxxxxxxxx
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Acked-by: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
fs/fs-writeback.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index fc9167e65287..869a34a48958 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -512,9 +512,14 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
/* find and pin the new wb */
memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
- if (memcg_css)
- isw->new_wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
+ if (memcg_css && !css_tryget(memcg_css))
+ memcg_css = NULL;
+ if (!memcg_css)
+ goto out_free;
+ isw->new_wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
+ css_put(memcg_css);
if (!isw->new_wb)
goto out_free;