linux-kernel - Re: [RFC] Printk deadlock in bpf trace called from scheduler context

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Message-ID: <20240729122726.GA33588@noisy.programming.kicks-ass.net>
Date: Mon, 29 Jul 2024 14:27:26 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Radoslaw Zielonek <radoslaw.zielonek@...il.com>
Cc: rostedt@...dmis.org, mhiramat@...nel.org,
	mathieu.desnoyers@...icios.com, mingo@...hat.com,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, song@...nel.org, jolsa@...nel.org,
	ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
	martin.lau@...ux.dev, eddyz87@...il.com, yonghong.song@...ux.dev,
	john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me,
	haoluo@...gle.com, mattbobrowski@...gle.com, qyousef@...alina.io,
	tiozhang@...iglobal.com, elver@...gle.com,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	bpf@...r.kernel.org
Subject: Re: [RFC] Printk deadlock in bpf trace called from scheduler context

On Mon, Jul 29, 2024 at 01:46:09PM +0200, Radoslaw Zielonek wrote:
> I am currently working on a syzbot-reported bug where bpf
> is called from trace_sched_switch. In this scenario, we are still within
> the scheduler context, and calling printk can create a deadlock.
> 
> I am uncertain about the best approach to fix this issue.

It's been like this forever, it doesn't need fixing, because tracepoints
shouldn't be doing printk() in the first place.

> Should we simply forbid such calls, or perhaps we should replace printk
> with printk_deferred in the bpf where we are still in scheduler context?

Not doing printk() is best.