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

[Guopeng Zhang]

在 2026/7/8 13:29, Tao Cui 写道:
> 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.
> 
hello，

Thanks for the review, and thanks for pointing out the overlap.

If Yu's series gets merged first, this patch can simply be dropped.

Thanks,
Guopeng
> 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>
>>
>