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Date:   Mon, 29 Mar 2021 13:19:07 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Daniel Thompson <daniel.thompson@linaro.org>
Cc:     Waiman Long <longman@redhat.com>, Ingo Molnar <mingo@redhat.com>,
        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>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Bharata B Rao <bharata@linux.vnet.ibm.com>,
        Phil Auld <pauld@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] sched/debug: Don't disable IRQ when acquiring
 sched_debug_lock
Message-ID: <YGG3q1LxtaF1/wc1@hirez.programming.kicks-ass.net>
References: <20210327232529.2349-1-longman@redhat.com>
 <20210329102345.6awrhtlrueqwhrpi@maple.lan>
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On Mon, Mar 29, 2021 at 11:23:45AM +0100, Daniel Thompson wrote:
> On Sat, Mar 27, 2021 at 07:25:28PM -0400, Waiman Long wrote:
> > The sched_debug_lock was used only in print_cpu().  The
> > print_cpu() function has two callers - sched_debug_show() and
> > sysrq_sched_debug_show(). Both of them are invoked by user action
> > (sched_debug file and sysrq-t). As print_cpu() won't be called from
> > interrupt context at all, there is no point in disabling IRQ when
> > acquiring sched_debug_lock.
> 
> This looks like it introduces a deadlock risk if sysrq-t triggers from an
> interrupt context. Has the behaviour of sysrq changed recently or will
> tools like MAGIC_SYSRQ_SERIAL still trigger from interrupt context?

Yeah, sysrq-t is very often interrupt context. The patch is clearly
bogus.