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Date:   Thu, 18 Aug 2022 17:22:53 +0200
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Christoph Lameter <cl@gentwo.de>, linux-kernel@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Pekka Enberg <penberg@kernel.org>, linux-mm@kvack.org
Subject: Re: [PATCH 1/9] slub: Make PREEMPT_RT support less convoluted
Message-ID: <Yv5ZTXMmk9LupRae@linutronix.de>
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On 2022-08-18 16:37:06 [+0200], Vlastimil Babka wrote:
> > The slub fastpath does not interfere with slow path operations and the
> 
> That's true on !PREEMPT_RT because a slowpath operation under
> local_lock_irqsave() will disable interrupts, so there can't be a
> fastpath operation in an interrupt handler appearing in the middle of a
> slowpath operation.
> 
> On PREEMPT_RT local_lock_irqsave() doesn't actually disable interrupts,
> so that can happen. IIRC we learned that the hard way when Mike
> Galbraith was testing early versions of my PREEMPT_RT changes for SLUB.

I think the point is that local_lock_irqsave() does not disable
preemption. So the lock owner (within the local_lock_irqsave() section)
can be preempted and another task can use the fast path which does not
involve the lock and things go boom from here.

Sebastian