Re: [Xen-devel] [PATCH 0/8] xen: don't let keyhandlers block indefinitely on locks

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.02.2020 19:38, Andrew Cooper wrote:
> On 13/02/2020 12:54, Juergen Gross wrote:
>> Keyhandlers dumping hypervisor information to the console often need
>> to take locks while accessing data. In order to not block in case of
>> system inconsistencies it is convenient to use trylock variants when
>> obtaining the locks. On the other hand a busy system might easily
>> encounter held locks, so this patch series is adding special trylock
>> variants with a timeout used by keyhandlers.
> 
> This is a backwards step.
> 
> Keyhandlers are for debugging purposes.  When debugging it is far more 
> important to get the requested data, than almost anything else.
> 
> The system will cope with a multi-second outage occurring approximately 
> never.  A person debugging who can't get the data has no chance of 
> fixing whatever problem they are looking for.
> 
> This series seems to be breaking the one critical usecase for 
> keyhandlers, to fix what - not let debugging get in the way of the 
> smooth running of the system?  A system in need of debugging in the 
> first place has bigger problems than needing to run smoothly.

I certainly accept what you say further up, but I don't think this
last statement is universally true. There may be a single guest in
trouble, which - to find out about its state - some debugging keys
may want issuing. Disturbing the host and all other guests for this
is not a good idea imo.

Jan

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