linux-kernel - Re: [PATCH RFC v1 1/2] rcu/tree: Add basic support for kfree

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Date:   Mon, 12 Aug 2019 09:12:34 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Byungchul Park <byungchul.park@....com>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Byungchul Park <max.byungchul.park@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Rao Shoaib <rao.shoaib@...cle.com>, kernel-team@...roid.com,
        kernel-team <kernel-team@....com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rcu <rcu@...r.kernel.org>, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH RFC v1 1/2] rcu/tree: Add basic support for kfree_rcu
 batching

On Mon, Aug 12, 2019 at 07:10:52PM +0900, Byungchul Park wrote:
> On Sun, Aug 11, 2019 at 04:49:39PM -0700, Paul E. McKenney wrote:
> > Maybe.  Note well that I said "potential issue".  When I checked a few
> > years ago, none of the uses of rcu_barrier() cared about kfree_rcu().
> > They cared instead about call_rcu() callbacks that accessed code or data
> > that was going to disappear soon, for example, due to module unload or
> > filesystem unmount.
> > 
> > So it -might- be that rcu_barrier() can stay as it is, but with changes
> > as needed to documentation.

Right, we should update the docs. Byungchul, do you mind sending a patch that
documents the rcu_barrier() behavior?

> > It also -might- be, maybe now or maybe some time in the future, that
> > there will need to be a kfree_rcu_barrier() or some such.  But if so,
> > let's not create it until it is needed.  For one thing, it is reasonably
> > likely that something other than a kfree_rcu_barrier() would really
> > be what was needed.  After all, the main point would be to make sure
> > that the old memory really was freed before allocating new memory.
> 
> Now I fully understand what you meant thanks to you. Thank you for
> explaining it in detail.
> 
> > But if the system had ample memory, why wait?  In that case you don't
> > really need to wait for all the old memory to be freed, but rather for
> > sufficient memory to be available for allocation.
> 
> Agree. Totally make sense.

Agreed, all makes sense.

thanks,

 - Joel

[snip]