linux-kernel - Re: [PATCH v4 1/2] sched: Fix UCLAMP_FLAG

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20210727143221.w3dxsc7ovc6cnce5@e107158-lin.cambridge.arm.com>
Date:   Tue, 27 Jul 2021 15:32:21 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Quentin Perret <qperret@...gle.com>
Cc:     mingo@...hat.com, peterz@...radead.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rickyiu@...gle.com, wvw@...gle.com,
        patrick.bellasi@...bug.net, xuewen.yan94@...il.com,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v4 1/2] sched: Fix UCLAMP_FLAG_IDLE setting

On 07/19/21 17:16, Quentin Perret wrote:
> The UCLAMP_FLAG_IDLE flag is set on a runqueue when dequeueing the last
> active task to maintain the last uclamp.max and prevent blocked util
> from suddenly becoming visible.
> 
> However, there is an asymmetry in how the flag is set and cleared which
> can lead to having the flag set whilst there are active tasks on the rq.
> Specifically, the flag is cleared in the uclamp_rq_inc() path, which is
> called at enqueue time, but set in uclamp_rq_dec_id() which is called
> both when dequeueing a task _and_ in the update_uclamp_active() path. As
> a result, when both uclamp_rq_{dec,ind}_id() are called from
> update_uclamp_active(), the flag ends up being set but not cleared,
> hence leaving the runqueue in a broken state.
> 
> Fix this by clearing the flag in update_uclamp_active() as well.
> 
> Fixes: e496187da710 ("sched/uclamp: Enforce last task's UCLAMP_MAX")
> Reported-by: Rick Yiu <rickyiu@...gle.com>
> Signed-off-by: Quentin Perret <qperret@...gle.com>
> ---

I've put a note that handling of this flag needs to be improved for the future.
But for now and FWIW, this LGTM:

Reviewed-by: Qais Yousef <qais.yousef@....com>

Thanks!

--
Qais Yousef