linux-kernel - Re: [PATCH] padata: use smp_mb in padata_reorder to avoid orphaned padata jobs

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Message-ID: <20190716111410.GN17989@gauss3.secunet.de>
Date:   Tue, 16 Jul 2019 13:14:10 +0200
From:   Steffen Klassert <steffen.klassert@...unet.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
CC:     Daniel Jordan <daniel.m.jordan@...cle.com>,
        <andrea.parri@...rulasolutions.com>, <boqun.feng@...il.com>,
        <paulmck@...ux.ibm.com>, <peterz@...radead.org>,
        <linux-arch@...r.kernel.org>, <linux-crypto@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] padata: use smp_mb in padata_reorder to avoid orphaned
 padata jobs

On Tue, Jul 16, 2019 at 06:04:47PM +0800, Herbert Xu wrote:
> On Mon, Jul 15, 2019 at 12:10:46PM -0400, Daniel Jordan wrote:
> >
> > I've been wrong before plenty of times, and there's nothing preventing this
> > from being one of those times :) , but in this case I believe what I'm showing
> > is correct.
> > 
> > The padata_do_serial call for a given job ensures padata_reorder runs on the
> > CPU that the job hashed to in padata_do_parallel, which is not necessarily the
> > same CPU as the one that padata_do_parallel itself ran on.
> 
> You're right.  I was taking the comment in the code at face value,
> never trust comments :)
> 
> While looking at the code in question, I think it is seriously
> broken.  For instance, padata_replace does not deal with async
> crypto at all.  It would fail miserably if the underlying async
> crypto held onto references to the old pd.

Hm, yes looks like that.

padata_replace should not call padata_free_pd() as long as the
refcount is not zero. Currenlty padata_flush_queues() will
BUG if there are references left.

Maybe we can fix it if we call padata_free_pd() from
padata_serial_worker() when it sent out the last object.