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writeback: fix obtain a reference to a freeing memcg css

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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: 682aa8e1a6 ("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>
This commit is contained in:
Muchun Song 2021-04-02 17:11:45 +08:00 • committed by Jan Kara
parent ce1b06c5f5
commit 8b0ed8443a
1 changed files with 7 additions and 2 deletions
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9
fs/fs-writeback.c
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@ -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;