Re: [PATCH] blk-cgroup: clear blkg->pd[] with WRITE_ONCE() in blkcg_deactivate_policy()

[Tao Cui]

FYI, this WRITE_ONCE() change is also in Yu Kuai's in-flight series that
factors the pd teardown loop into a helper:

  https://lore.kernel.org/all/20260625025739.2459651-5-yukuai@xxxxxxxxxx/

That series isn't merged yet (v3, 2026-06-25), so this minimal Fixes
patch is still useful in the meantime -- just flagging the overlap.

在 2026/7/8 13:10, Tao Cui 写道:
> 
> 
> 在 2026/7/7 20:58, Guopeng Zhang 写道:
>> From: Guopeng Zhang <zhangguopeng@xxxxxxxxxx>
>>
>> blkcg_activate_policy() installs blkg->pd[] entries with WRITE_ONCE()
>> and also uses WRITE_ONCE() when clearing them on its error path.
>> blkg_to_pd() is used by RCU readers and reads the same array with
>> READ_ONCE().
>>
>> blkcg_deactivate_policy() clears the entry with a plain store. Use
>> WRITE_ONCE() there as well.
>>
>> Fixes: 56cc24f59c14 ("blk-cgroup: don't nest queue_lock under rcu in blkcg_print_blkgs()")
>> Signed-off-by: Guopeng Zhang <zhangguopeng@xxxxxxxxxx>
>> ---
>>  block/blk-cgroup.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
>> index d2a1f5903f24..a1dd69f99f5c 100644
>> --- a/block/blk-cgroup.c
>> +++ b/block/blk-cgroup.c
>> @@ -1691,7 +1691,7 @@ void blkcg_deactivate_policy(struct gendisk *disk,
>>  			if (blkg->pd[pol->plid]->online && pol->pd_offline_fn)
>>  				pol->pd_offline_fn(blkg->pd[pol->plid]);
>>  			pol->pd_free_fn(blkg->pd[pol->plid]);
>> -			blkg->pd[pol->plid] = NULL;
>> +			WRITE_ONCE(blkg->pd[pol->plid], NULL);
>>  		}
>>  		spin_unlock(&blkcg->lock);
>>  	}
> 
> Reviewed-by: Tao Cui <cuitao@xxxxxxxxxx>
>