linux-kernel - Re: [PATCH v2 1/5] padata: make flushing work with async users

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Date:   Wed, 18 Sep 2019 16:37:30 -0400
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/5] padata: make flushing work with async users

On Thu, Sep 05, 2019 at 06:37:56PM -0400, Daniel Jordan wrote:
> On Thu, Sep 05, 2019 at 02:17:35PM +1000, Herbert Xu wrote:
> > I don't think waiting is an option.  In a pathological case the
> > hardware may not return at all.  We cannot and should not hold off
> > CPU hotplug for an arbitrary amount of time when the event we are
> > waiting for isn't even occuring on that CPU.
> 
> Ok, I hadn't considered hardware not returning.
> 
> > I don't think flushing is needed at all.  All we need to do is
> > maintain consistency before and after the CPU hotplug event.
> 
> I could imagine not flushing would work for replacing a pd.  The old pd could
> be freed by whatever drops the last reference and the new pd could be
> installed, all without flushing.
> 
> In the case of freeing an instance, though, padata needs to wait for all the
> jobs to complete so they don't use the instance's data after it's been freed.
> Holding the CPU hotplug lock isn't necessary for this, though, so I think we're
> ok to wait here.

[FYI, I'm currently on leave until mid-October and will return to this series
then.]