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

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Date:   Tue, 11 Aug 2020 21:39:10 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     paulmck@...nel.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

Thomas Gleixner <tglx@...utronix.de> writes:
> "Paul E. McKenney" <paulmck@...nel.org> writes:
>> On Tue, Aug 11, 2020 at 04:44:21PM +0200, Thomas Gleixner wrote:
>>> Now RCU creates a new thing which enforces to make page allocation in
>>> atomic context possible on RT. What for?
>>> 
>>> What's the actual use case in truly atomic context for this new thing on
>>> an RT kernel?
>>
>> It is not just RT kernels.  CONFIG_PROVE_RAW_LOCK_NESTING=y propagates
>> this constraint to all configurations, and a patch in your new favorite
>> subsystem really did trigger this lockdep check in a non-RT kernel.
>>
>>> The actual RCU code disabling interrupts is an implementation detail
>>> which can easily be mitigated with a local lock.
>>
>> In this case, we are in raw-spinlock context on entry to kfree_rcu().
>
> Where?

And aside of the where, wasn't kfree_rcu() from within raw spinlock held
regions possible all the time? Either I'm missing something or you are
fundamentally changing RCU internals. kfree_rcu() saved RT in various
ways where invoking kfree() was just not an option. Confused...

Thanks,

        tglx