On 11/30/22 10:16, Michal Koutný wrote:
On Tue, Nov 29, 2022 at 03:34:00PM -0500, Waiman Long <longman@xxxxxxxxxx> wrote:
The reproducing system can no longer produce a warning with this patch.
All the runnable block/0* tests including block/027 were run successfully
without failure.
Thanks for the test!
@@ -1088,7 +1088,15 @@ static void blkcg_destroy_blkgs(struct blkcg *blkcg)
might_sleep();
- css_get(&blkcg->css);
+ /*
+ * blkcg_destroy_blkgs() shouldn't be called with all the blkcg
+ * references gone and rcu_read_lock not held.
+ */
+ if (!css_tryget(&blkcg->css)) {
+ WARN_ON_ONCE(!rcu_read_lock_held());
+ return;
+ }
As I followed the previous discussion, the principle is that obtaining a
reference or being inside an RCU read section is sufficient.
Consequently, I'd expect the two situations handled equally but here the
no-ref but RCU bails out. (Which is OK because blkg_list must be empty?)
However, the might_sleep() in (non-sleepable) RCU reader section combo
makes me wary anyway (not with the early return but tools would likely
complain).
All in all, can't the contract of blkcg_destroy_blkgs() declare that
a caller must pass blkcg with a valid reference? (The body of
blkcg_destroy_blkgs then wouldn't need to get neither put the inner
reference).
You are right. I should have pushed the might_sleep down(). Will post a
new version to fix that.
Thanks,
Longman
