Hi,
在 2023/12/19 9:28, Ming Lei 写道:
blkg_lookup() is called with either queue_lock or rcu read lock, so
use rcu_dereference_check(lockdep_is_held(&q->queue_lock)) for
retrieving 'blkg', which way models the check exactly for covering
queue lock or rcu read lock.
Fix lockdep warning of "block/blk-cgroup.h:254 suspicious rcu_dereference_check() usage!"
from blkg_lookup().
Tested-by: Changhui Zhong <czhong@xxxxxxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
---
block/blk-cgroup.h | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h
index fd482439afbc..b927a4a0ad03 100644
--- a/block/blk-cgroup.h
+++ b/block/blk-cgroup.h
@@ -252,7 +252,8 @@ static inline struct blkcg_gq *blkg_lookup(struct blkcg *blkcg,
if (blkcg == &blkcg_root)
return q->root_blkg;
- blkg = rcu_dereference(blkcg->blkg_hint);
+ blkg = rcu_dereference_check(blkcg->blkg_hint,
+ lockdep_is_held(&q->queue_lock));
This patch itself is correct, and in fact this is a false positive
warning.
I noticed that commit 83462a6c971c ("blkcg: Drop unnecessary RCU read
[un]locks from blkg_conf_prep/finish()") drop rcu_read_lock/unlock()
because 'queue_lock' is held. This is correct, however you add this back
for tg_conf_updated() later in commit 27b13e209ddc ("blk-throttle: fix
lockdep warning of "cgroup_mutex or RCU read lock required!"") because
rcu_read_lock_held() from blkg_lookup() is triggered. And this patch is
again another use case cased by commit 83462a6c971c.
I just wonder, with the respect of rcu implementation, is it possible to
add preemptible() check directly in rcu_read_lock_held() to bypass all
this kind of false positive warning?
Thanks,
Kuai
if (blkg && blkg->q == q)
return blkg;
