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

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: <CAMbhsRQfHCJ=hY=hyX2TJLRAArX7iAhDdxTkaP9hjC5nR5U5vA@mail.gmail.com>
Date:	Mon, 28 Apr 2014 10:44:49 -0700
From:	Colin Cross <ccross@...roid.com>
To:	Daniel Thompson <daniel.thompson@...aro.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	kgdb-bugreport@...ts.sourceforge.net,
	Jason Wessel <jason.wessel@...driver.com>,
	"patches@...aro.org" <patches@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	lkml <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>,
	Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [RFC v3 1/9] sysrq: Implement __handle_sysrq_nolock to avoid
 recursive locking in kdb

On Mon, Apr 28, 2014 at 3:24 AM, Daniel Thompson
<daniel.thompson@...aro.org> wrote:
> 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.
>
>

One of the advantages of FIQ debugger is that it can be triggered from
an FIQ (NMI for those in x86 land), and Jason and I have discussed
using FIQs for kgdb to allow interrupting cpus stuck in critical
sections.  If that gets implemented the above assumption will no
longer be correct.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/