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Date: Tue, 22 Sep 2026 09:18:16 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: John Stultz <jstultz@google.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>, Hui Su <sh_def@163.com>,
	Ingo Molnar <mingo@redhat.com>, Juri Lelli <juri.lelli@redhat.com>,
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	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Valentin Schneider <vschneid@redhat.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 1/1] sched/proxy_exec: detect cycles in proxy walks
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On Mon, Sep 21, 2026 at 11:06:52PM -0700, John Stultz wrote:

> > So if we are eventually planning on returning -EDEADLK to userspace
> > (context: https://lore.kernel.org/lkml/20260917153649.GK4121339@noisy.programming.kicks-ass.net/)
> > do we care where the chain starts or can we return -EDEADLK anywhere
> > in the chain?
> 
> So returning EDEADLK to userland seems like a new feature to me, as I
> don't believe existing futexes (which can still have dependency cycles
> or just ABA style usage) currently do this.

  git grep -e DEADLK -e DEADLOCK kernel/{futex,locking}

We definitely return -EDEADLK for PI futexes and rt_mutex. And given the
whole proxy thing hard relies on the block graph being acyclic, it
seems rather mandatory we enforce this before exposing it to userspace.