Re: [RFC v3 1/9] sysrq: Implement __handle_sysrq_nolock to avoid recursive locking in kdb

[Daniel Thompson <daniel.thompson@linaro.org>]

On 25/04/14 17:45, Steven Rostedt wrote:
> On Fri, 25 Apr 2014 17:29:22 +0100
> Daniel Thompson <daniel.thompson@linaro.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.