linux-kernel - Re: [RFC v3 1/9] sysrq: Implement __handle_sysrq

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Message-ID: <535E2C5A.9090702@linaro.org>
Date:	Mon, 28 Apr 2014 11:24:26 +0100
From:	Daniel Thompson <daniel.thompson@...aro.org>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	kgdb-bugreport@...ts.sourceforge.net,
	Jason Wessel <jason.wessel@...driver.com>, patches@...aro.org,
	linaro-kernel@...ts.linaro.org, linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	John Stultz <john.stultz@...aro.org>,
	Anton Vorontsov <anton.vorontsov@...aro.org>,
	Colin Cross <ccross@...roid.com>, kernel-team@...roid.com
Subject: Re: [RFC v3 1/9] sysrq: Implement __handle_sysrq_nolock to avoid
 recursive locking in kdb

On 25/04/14 17:45, Steven Rostedt wrote:
> On Fri, 25 Apr 2014 17:29:22 +0100
> Daniel Thompson <daniel.thompson@...aro.org> wrote:
> 
>> If kdb is triggered using SysRq-g then any use of the sr command results
>> in the SysRq key table lock being recursively acquired, killing the debug
>> session. That patch resolves the problem by introducing a _nolock
>> alternative for __handle_sysrq.
>>
>> Strictly speaking this approach risks racing on the key table when kdb is
>> triggered by something other than SysRq-g however in that case any other
>> CPU involved should release the spin lock before kgdb parks the slave
>> CPUs.
> 
> Is that case documented somewhere in the code comments?

Perhaps not near enough to the _nolock but the primary bit of comment is
here (and in same file as kdb_sr).
--- cut here ---
 * kdb_main_loop - After initial setup and assignment of the
 *	controlling cpu, all cpus are in this loop.  One cpu is in
 *	control and will issue the kdb prompt, the others will spin
 *	until 'go' or cpu switch.
--- cut here ---

The mechanism kgdb uses to quiesce other CPUs means other CPUs cannot be
in irqsave critical sections.


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