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

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Date:   Tue, 11 Aug 2020 09:02:40 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
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 Tue, Aug 11, 2020 at 05:43:16PM +0200, Thomas Gleixner wrote:
> "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?

Some BPF code that needs to process and free a list.  As noted above,
this is a patch rather than something that is already in mainline.
Not surprising, though, given call_rcu() invocations in similar contexts.

Yes, we can perhaps rework all current and future callers to avoid
invoking both call_rcu() and kfree_rcu() from raw atomic context, but
the required change to permit this is quite a bit simpler.

							Thanx, Paul