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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: 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:
parent
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commit
8b0ed8443a
1 changed files with 7 additions and 2 deletions
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@ -508,9 +508,14 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
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/* find and pin the new wb */
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rcu_read_lock();
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memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
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if (memcg_css)
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isw->new_wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
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if (memcg_css && !css_tryget(memcg_css))
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memcg_css = NULL;
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rcu_read_unlock();
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if (!memcg_css)
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goto out_free;
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isw->new_wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
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css_put(memcg_css);
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if (!isw->new_wb)
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goto out_free;
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