linux-kernel - Re: synchronize_rcu_expedited gets stuck in hotplug path

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: <Yece9mH3nzwGxar6@slm.duckdns.org>
Date:   Tue, 18 Jan 2022 10:11:34 -1000
From:   Tejun Heo <tj@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Mukesh Ojha <quic_mojha@...cinc.com>,
        lkml <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>, jiangshanlai@...il.com
Subject: Re: synchronize_rcu_expedited gets stuck in hotplug path

Hello,

On Tue, Jan 18, 2022 at 12:06:46PM -0800, Paul E. McKenney wrote:
> Interesting.  Adding Tejun and Lai on CC for their perspective.
> 
> As you say, the incoming CPU invoked synchronize_rcu_expedited() which
> in turn invoked queue_work().  By default, workqueues will of course
> queue that work on the current CPU.  But in this case, the CPU's bit
> is not yet set in the cpu_active_mask.  Thus, a workqueue scheduled on
> the incoming CPU won't be invoked until CPUHP_AP_ACTIVE, which won't
> be reached until after the grace period ends, which cannot happen until
> the workqueue handler is invoked.
> 
> I could imagine doing something as shown in the (untested) patch below,
> but first does this help?
> 
> If it does help, would this sort of check be appropriate here or
> should it instead go into workqueues?

Maybe it can be solved by rearranging the hotplug sequence but it's fragile
to schedule per-cpu work items from hotplug paths. Maybe the whole issue can
be side-stepped by making synchronize_rcu_expedited() use unbound workqueue
instead? Does it require to be per-cpu?

Thanks.

-- 
tejun