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Subject: Re: [RFC v2 33/34] mm, slub: use migrate_disable() on PREEMPT_RT
From:   Vlastimil Babka <vbabka@suse.cz>
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Christoph Lameter <cl@linux.com>,
        David Rientjes <rientjes@google.com>,
        Pekka Enberg <penberg@kernel.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Mel Gorman <mgorman@techsingularity.net>,
        Jesper Dangaard Brouer <brouer@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Jann Horn <jannh@google.com>
References: <20210609113903.1421-1-vbabka@suse.cz>
 <20210609113903.1421-34-vbabka@suse.cz>
 <c553c26f-1f9c-e5ab-b71b-b2b5eeb59ad2@suse.cz>
 <20210614111619.l3ral7tt2wasvlb4@linutronix.de>
 <390fc59e-17ed-47eb-48ff-8dae93c9a718@suse.cz>
Message-ID: <0edf0478-17ed-4120-22e1-b61ad9bddeec@suse.cz>
Date:   Mon, 14 Jun 2021 14:54:30 +0200
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On 6/14/21 1:33 PM, Vlastimil Babka wrote:
> On 6/14/21 1:16 PM, Sebastian Andrzej Siewior wrote:
> But now that I dig into this in detail, I can see there might be another
> instance of this imbalance bug, if CONFIG_PREEMPTION is disabled, but
> CONFIG_PREEMPT_COUNT is enabled, which seems to be possible in some debug
> scenarios. Because then preempt_disable()/preempt_enable() still manipulate the
> preempt counter and compiling them out in __slab_alloc() will cause imbalance.
> 
> So I think the guards in __slab_alloc() should be using CONFIG_PREEMPT_COUNT
> instead of CONFIG_PREEMPT to be correct on all configs. I dare not remove them
> completely :)

Yep, it's possible to get such scenario with PREEMPT_VOLUNTARY plus
PROVE_LOCKING - CONFIG_PREEMPTION is disabled but CONFIG_PREEMPT_COUNT is
enabled, and RCU then complains in the page allocator due to the unpaired
preempt_disable() before entering it.

I've pushed a new branch revision with this fixed:

https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/linux.git/log/?h=slub-local-lock-v2r3