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

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Date:   Fri, 14 Aug 2020 21:33:47 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     paulmck@...nel.org, Peter Zijlstra <peterz@...radead.org>
Cc:     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 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?

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.

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.

Thanks,

        tglx