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>

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index e91980f..d52c291 100644 (file)
--- 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;