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

[Colin Cross]

On Mon, Apr 28, 2014 at 3:24 AM, Daniel Thompson
<daniel.thompson@xxxxxxxxxx> wrote:
> On 25/04/14 17:45, Steven Rostedt wrote:
>> On Fri, 25 Apr 2014 17:29:22 +0100
>> Daniel Thompson <daniel.thompson@xxxxxxxxxx> 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.
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