lkml.org 
[lkml]   [2018]   [Apr]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH] blkcg: not hold blkcg lock when deactivating policy.
From
Date
On 4/18/18 3:18 AM, jiang.biao2@zte.com.cn wrote:
> Hi,
>>> Il giorno 17 apr 2018, alle ore 09:10, Jiang Biao <jiang.biao2@zte.com.cn> ha scritto:
>>>
>>> As described in the comment of blkcg_activate_policy(),
>>> *Update of each blkg is protected by both queue and blkcg locks so
>>> that holding either lock and testing blkcg_policy_enabled() is
>>> always enough for dereferencing policy data.*
>>> with queue lock held, there is no need to hold blkcg lock in
>>> blkcg_deactivate_policy(). Similar case is in
>>> blkcg_activate_policy(), which has removed holding of blkcg lock in
>>> commit 4c55f4f9ad3001ac1fefdd8d8ca7641d18558e23.
>>>
>>
>> Hi,
>> 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.

I've applied your patch, thanks.

--
Jens Axboe

\
 
 \ /
  Last update: 2018-04-18 16:41    [W:0.031 / U:0.308 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site