linux-kernel - Re: [PATCH v3 3/3] sched_ext: Allow scx_bpf_reenqueue

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aTruf0rK94ujYH0f@slm.duckdns.org>
Date: Thu, 11 Dec 2025 06:17:03 -1000
From: Tejun Heo <tj@...nel.org>
To: Kuba Piecuch <jpiecuch@...gle.com>
Cc: David Vernet <void@...ifault.com>,
	Andrea Righi <andrea.righi@...ux.dev>,
	Changwoo Min <changwoo@...lia.com>, linux-kernel@...r.kernel.org,
	sched-ext@...ts.linux.dev, Peter Zijlstra <peterz@...radead.org>,
	Wen-Fang Liu <liuwenfang@...or.com>
Subject: Re: [PATCH v3 3/3] sched_ext: Allow scx_bpf_reenqueue_local() to be
 called from anywhere

Hello, Kuba.

On Thu, Dec 11, 2025 at 02:24:04PM +0000, Kuba Piecuch wrote:
> Since we're doing a direct dispatch, the user scheduler task will be
> inserted into the dispatch queue in enable_task_scx(), without dropping the rq
> lock between example_enqueue() and the insertion, which means reenq_local()
> will run afterwards (since it's deferred using irq_work), removing all tasks
> from the DSQ, including the userspace scheduler.
> 
> A similar problem arises even if we don't do direct dispatch and drop the rq
> lock after example_enqueue(): since dispatching and reenq_local() are deferred
> using different irq_work entries, and irq_work_run() processes entries from
> newest to oldest, dispatching will be handled before reenq_local(), yielding
> the same result.

Oh yeah, the asynchronity can become pretty confusing.

> The user may be unaware of this behavior (it's not mentioned anywhere) and
> expect the reenqueue to happen before dispatching the new task.
> 
> I think at the very least we should make users aware of this in the comment
> for scx_bpf_reenqueue_local___v2().

Documentation is always helpful but I wonder whether this can be improved by
the reenqueue function capturing the dsq seq number and re-enqueueing only
the ones that were enqueued before the reenqueue was called.

Thanks.

-- 
tejun