linux-kernel - Re: [PATCH] cgroup/cpuset: fix circular locking dependency

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Date:   Tue, 16 Jan 2018 08:27:30 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Prateek Sood <prsood@...eaurora.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, avagin@...il.com,
        mingo@...nel.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, sramana@...eaurora.org,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] cgroup/cpuset: fix circular locking dependency

Hello, Prateek.

On Mon, Jan 15, 2018 at 05:32:18PM +0530, Prateek Sood wrote:
> My understanding of WQ_MEM_RECLAIM was that it needs to be used for
> cases where memory pressure could cause deadlocks.

Yes, that is the primary role; however, there are a couple places
where we need it to isolate a low level subsystem's forward progress
from the dependencies of the dynanic worker pool management.

> In this case it does not seem to be a memory pressure issue.
> Overloading WQ_MEM_RECLAIM usage for solution to another problem
> is the correct approach?

In general, we don't face this issue but for things like RCU, I do
believe this is the right approach.  The name of the flag is a bit of
misnomer for those cases as it only describes one of the dependencies
of worker pool management.  I don't think it's an actual problem given
that vast majority of the usages are protecting against memory
dependency.

> This scenario can be resolved by using WQ_MEM_RECLAIM and a separate
> workqueue for rcu. But there seems to be a possibility in future if
> any cpu hotplug callbacks use other predefined workqueues which do not
> have WQ_MEM_RECLAIM option.

We want to isolate from this sort of dependencies anyway, especially
how widely and deeply we depend on RCU these days.  Maybe we will
develop a justified case in the future but at least up until now every
case around this issue seems to be something to be fixed on their own.

Thanks.

-- 
tejun