On 11/30/22 8:16?AM, Michal Koutn? 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).
Totally agree, the proposed patch feels more like a hacky workaround
rather than a true solution. Either the contract should be that it's
ALWAYS entered with RCU lock held and hence the tryget is fine, OR that
a reference always is held when entered.
I'm going to revert the offending patch for now, and then we can queue
up a proper patch when that exists.
--
Jens Axboe
