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Date: Sat, 29 Aug 2026 15:49:22 +0100
From: Matthew Wilcox <willy@infradead.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: David Stevens <stevensd@google.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Thomas Gleixner <tglx@kernel.org>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
	"H . Peter Anvin" <hpa@zytor.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dave Chinner <david@fromorbit.com>, Qi Zheng <qi.zheng@linux.dev>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Muchun Song <muchun.song@linux.dev>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Valentin Schneider <vschneid@redhat.com>,
	K Prateek Nayak <kprateek.nayak@amd.com>,
	Uladzislau Rezki <urezki@gmail.com>,
	David Hildenbrand <david@kernel.org>,
	Lorenzo Stoakes <ljs@kernel.org>,
	"Liam R . Howlett" <liam@infradead.org>,
	Vlastimil Babka <vbabka@kernel.org>,
	Mike Rapoport <rppt@kernel.org>,
	Suren Baghdasaryan <surenb@google.com>,
	Michal Hocko <mhocko@suse.com>, Kees Cook <kees@kernel.org>,
	Clark Williams <clrkwllms@kernel.org>, suleiman@google.com,
	linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-mm@kvack.org, linux-rt-devel@lists.linux.dev
Subject: Re: [RFC 06/10] Reclaim memory from blocked kernel stacks
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References: <20260827232948.2520558-1-stevensd@google.com>
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 <20260828133620._x2XfJR_@linutronix.de>
 <20260828135947.GU776954@noisy.programming.kicks-ass.net>
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In-Reply-To: <20260829084901.GA776954@noisy.programming.kicks-ass.net>

On Sat, Aug 29, 2026 at 10:49:01AM +0200, Peter Zijlstra wrote:
> > On PREEMPT_RT, it is necessary to skip the call to
> > alloc_pages_nolock_noprof() from under pi_lock, since with that
> > configuration spin_trylock can end up needing to take pi_lock. But at
> > least on !PREEMPT_RT, there is no risk of deadlock.
> 
> Yeah, which puts the lie to that horrific hack Alexei did. We should
> probably teach lockdep about spin_trylock() not being safe and see the
> house of cards crumble.

Why is spin_trylock() unsafe on PREEMPT_RT?  It's not intuitive since
one can mutex_trylock() in interrupt context or under spinlock.