Re: [PATCH v6] workqueue: Fix edge cases for calc of pool's cpumask

[Tejun Heo <tj@kernel.org>]

Hello, Michael.

On Thu, Jul 27, 2017 at 02:07:53PM -0500, Michael Bringmann wrote:
> The problem lies with the ordering of events with respect to the order in
> which we add (or remove) CPUs to NUMA systems, and make use of that knowledge.

Isn't the root cause that the upper layers including workqueue expect
cpu <-> node mapping to be static but powerpc doesn't follow that?  I
don't get why ordering matters here.

> The CPUs present are assigned to nodes, and workqueues and their infrastructure
> are created to use the CPUs in a node.  Workqueues are created at boot time
> and updated or created as CPUs are added or removed.  However, there is little
> or no synchronization or ordering of these events, and the data structures

What I meant was that there's no synchronization construct protection
cpu <-> node mapping.  If arch code changes it during hot plug, it's
changing it underneath anybody who might be using that association.

> mapping CPUs to nodes may not be updated before the workqueue infrastructure
> is built for a node.  Thus we have the possibility of an invalid CPU mask
> attribute being attached to a newly created workqueue before the CPUs have
> been properly registered and published to a node.
> 
> This patch attempts to provide a partial ordering of events within workqueue
> by delaying the use of newly calculated CPU masks as the value for a workqueue
> attribute until they have valid content.  Instead the workqueue code must delay
> creating new workqueues until this function succeeds, or it can use a previously
> calculated cpumask attribute that is known to be valid.
> 
> This patch attempts to ensure that a valid, usable cpumask is used to set up
> newly created pools for workqueues.  This patch provides a fix for NUMA systems
> which can add/subtract processors dynamically.  The patch is expected to be an
> intermediate one while developers find and correct any underlying issues.

And what this patch does is adding a bandaid so that we at least don't
crash immediately when this condition triggers until the arch code can
be fixed properly.

Thanks.

-- 
tejun