Re: [PATCH] blkcg: not hold blkcg lock when deactivating policy.

[Jens Axboe]

On 4/18/18 6:54 PM, jiang.biao2@xxxxxxxxxx wrote:
>>>> by chance, did you check whether this may cause problems with bfq,
>>>> being the latter not protected by the queue lock as cfq?
>>> Checked the bfq code, bfq seems never used blkcg lock derectly, and
>>> update of blkg in the common code is protected by both queue and
>>> blkcg locks, so IMHO this patch would not introduce any new problem
>>> with bfq, even though bfq is not protected by queue lock.
>>> On the other hand, the locks (queue lock/blkcg lock) used to protected
>>> the update of blkg seems a bit too heavyweight, especially the queue lock
>>> which is used too widely may cause races with other contexts. I wonder
>>> if there is any way to ease the case? e.g. add a new lock for blkg's own.:)
>>
>> It might make sense to lock it separately, but I would not worry
>> about it unless it shows up as hot in your testing.
> Actually, we've met a triggering of nmi_watchdog, blocked at the queue lock
> in blkcg_print_blkgs(), caused by the slow serial console and too many printks.
> Related discussion is here,
> https://bugzilla.kernel.org/show_bug.cgi?id=199003
> Even though it's not caused by the queue lock directly, it would not happen
> without using queue lock. The queue lock is big and used too widely, using it
> would intensify the race, so we're trying to understand the locks using in blkg,
> and maybe could improve the situation.

The queue lock is only used widely on non blk-mq, where it is the only
lock really. Doing serial IO under a spinlock is always going to suck,
regardless of how contended it is.

-- 
Jens Axboe