linux-kernel - Re: [RFC-PATCH 1/2] mm: Add __GFP_NO

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Date:   Fri, 14 Aug 2020 13:41:40 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Uladzislau Rezki <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Joel Fernandes <joel@...lfernandes.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

On Fri, Aug 14, 2020 at 09:33:47PM +0200, Thomas Gleixner wrote:
> On Fri, Aug 14 2020 at 11:02, Paul E. McKenney wrote:
> > On Fri, Aug 14, 2020 at 07:49:24PM +0200, Peter Zijlstra wrote:
> >> On Fri, Aug 14, 2020 at 09:11:06AM -0700, Paul E. McKenney wrote:
> >> > Just to make sure we are talking about the same thing, please see below
> >> > for an untested patch that illustrates how I was interpreting your words.
> >> > Was this what you had in mind?
> >> 
> >> No, definitely not.
> >> 
> >> Also, since we used to be able to use call_rcu() _everywhere_, including
> >> under zone->lock, how's that working with you calling the
> >> page-allocating from it?
> >
> > Indeed, that is exactly the problem we are trying to solve.
> 
> Wait a moment. Why are we discussing RT induced raw non raw lock
> ordering at all?

Because we like to argue?  (Sorry, couldn't resist.)

> Whatever kernel you variant you look at this is not working:
> 
>   lock(zone)  call_rcu() lock(zone)
> 
> It's a simple recursive dead lock, nothing else.

You are of course absolutely correct.

> And that enforces the GFP_NOLOCK allocation mode or some other solution
> unless you make a new rule that calling call_rcu() is forbidden while
> holding zone lock or any other lock which might be nested inside the
> GFP_NOWAIT zone::lock held region.

Again, you are correct.  Maybe the forecasted weekend heat will cause
my brain to hallucinate a better solution, but in the meantime, the
GFP_NOLOCK approach looks good from this end.

							Thanx, Paul