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Date:   Tue, 5 May 2020 09:23:35 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Tejun Heo <tj@...nel.org>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, cgroups@...r.kernel.org,
        Vlad Dmitriev <vvd@...com>
Subject: Re: [block/for-5.7] iocost: protect iocg->abs_vdebt with
 iocg->waitq.lock

On 5/4/20 5:27 PM, Tejun Heo wrote:
> Subject: iocost: protect iocg->abs_vdebt with iocg->waitq.lock
> 
> abs_vdebt is an atomic_64 which tracks how much over budget a given cgroup
> is and controls the activation of use_delay mechanism. Once a cgroup goes
> over budget from forced IOs, it has to pay it back with its future budget.
> The progress guarantee on debt paying comes from the iocg being active -
> active iocgs are processed by the periodic timer, which ensures that as time
> passes the debts dissipate and the iocg returns to normal operation.
> 
> However, both iocg activation and vdebt handling are asynchronous and a
> sequence like the following may happen.
> 
> 1. The iocg is in the process of being deactivated by the periodic timer.
> 
> 2. A bio enters ioc_rqos_throttle(), calls iocg_activate() which returns
>    without anything because it still sees that the iocg is already active.
> 
> 3. The iocg is deactivated.
> 
> 4. The bio from #2 is over budget but needs to be forced. It increases
>    abs_vdebt and goes over the threshold and enables use_delay.
> 
> 5. IO control is enabled for the iocg's subtree and now IOs are attributed
>    to the descendant cgroups and the iocg itself no longer issues IOs.
> 
> This leaves the iocg with stuck abs_vdebt - it has debt but inactive and no
> further IOs which can activate it. This can end up unduly punishing all the
> descendants cgroups.
> 
> The usual throttling path has the same issue - the iocg must be active while
> throttled to ensure that future event will wake it up - and solves the
> problem by synchronizing the throttling path with a spinlock. abs_vdebt
> handling is another form of overage handling and shares a lot of
> characteristics including the fact that it isn't in the hottest path.
> 
> This patch fixes the above and other possible races by strictly
> synchronizing abs_vdebt and use_delay handling with iocg->waitq.lock.

Applied, thanks.

-- 
Jens Axboe

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