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Date: Thu, 2 Jul 2026 20:24:06 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Andrea Righi <arighi@nvidia.com>
Cc: Tejun Heo <tj@kernel.org>, David Vernet <void@manifault.com>,
	Changwoo Min <changwoo@igalia.com>,
	John Stultz <jstultz@google.com>, Ingo Molnar <mingo@redhat.com>,
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	Christian Loehle <christian.loehle@arm.com>,
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	Aiqun Yu <aiqun.yu@oss.qualcomm.com>, Shuah Khan <shuah@kernel.org>,
	sched-ext@lists.linux.dev, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 02/12] sched/core: Skip put_prev_task/set_next_task
 re-entry for sched_ext donors
Message-ID: <20260702182406.GM751831@noisy.programming.kicks-ass.net>
References: <20260702171909.1994478-1-arighi@nvidia.com>
 <20260702171909.1994478-3-arighi@nvidia.com>
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In-Reply-To: <20260702171909.1994478-3-arighi@nvidia.com>

On Thu, Jul 02, 2026 at 07:09:18PM +0200, Andrea Righi wrote:
> In __schedule(), the proxy-exec donor-stabilization block calls
> put_prev_task() and set_next_task() when rq->donor == prev_donor and
> prev != next.
> 
> For sched_ext tasks, re-entering set_next_task_scx() for a donor that
> has already been seen by BPF ops.running via the normal pick path causes
> issues. It fires SCX_CALL_OP_TASK(sch, running, rq, donor) a second
> time, and sch->ops dispatch can land on a vtable slot in a state that
> yields a NULL function pointer or corrupts the stack.
> 
> Fix this by skipping the put_prev_task/set_next_task re-entry when the
> donor is in the ext_sched_class, since sched_ext tracks curr/donor
> itself.

This really sounds like a bug in ext; how is this different from the
sched_change pattern doing a put/set cycle?