linux-kernel - Re: [RFC][PATCH 0/5] perf/tracing/cpuhotplug: Fix locking order

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Message-ID: <20170512194956.GH4626@worktop.programming.kicks-ass.net>
Date:   Fri, 12 May 2017 21:49:56 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [RFC][PATCH 0/5] perf/tracing/cpuhotplug: Fix locking order

On Fri, May 12, 2017 at 01:15:44PM -0400, Steven Rostedt wrote:
>  2) Allow for get_online_cpus() to nest

So Thomas and me have been avoiding doing this.

In general we avoid nested locking in the kernel. Nested locking makes
an absolute mockery of locking rules and what all gets protected.

Yes, its much easier.. But we managed to kill the BKL, so surely we can
fix the hotplug lock too, right ;-)