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authorMuchun Song <songmuchun@bytedance.com>2021-04-02 17:11:45 +0800
committerJan Kara <jack@suse.cz>2021-06-28 18:35:02 +0200
commit8b0ed8443ae6458786580d36b7d5f8125535c5d4 (patch)
tree4cb9fc32fd24cb332daeb2cbd6d0715d43a06828 /fs/fs-writeback.c
parentce1b06c5f5e7fd2af8bc89323d9eb3102d8e3fe2 (diff)
downloadlwn-8b0ed8443ae6458786580d36b7d5f8125535c5d4.tar.gz
lwn-8b0ed8443ae6458786580d36b7d5f8125535c5d4.zip
writeback: fix obtain a reference to a freeing memcg css
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. rcu_read_lock() memcg_css = css_from_id() wb_get_create(memcg_css) cgwb_create(memcg_css) // css_get can change the ref counter from 0 back to 1 css_get(memcg_css) rcu_read_unlock() 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@bytedance.com Signed-off-by: Muchun Song <songmuchun@bytedance.com> Acked-by: Michal Hocko <mhocko@suse.com> Acked-by: Tejun Heo <tj@kernel.org> Signed-off-by: Jan Kara <jack@suse.cz>
Diffstat (limited to 'fs/fs-writeback.c')
-rw-r--r--fs/fs-writeback.c9
1 files changed, 7 insertions, 2 deletions
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index e91980f49388..d52c291d7bcd 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -508,9 +508,14 @@ 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)
- isw->new_wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
+ if (memcg_css && !css_tryget(memcg_css))
+ memcg_css = NULL;
rcu_read_unlock();
+ 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;