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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.
The only thing which should happen to improve system stability is for 
keyhandlers to disable the system watchdog while they are running, in 
case they happen to run for seconds of wallclock time. This is an issue 
which isn't addressed by the series, because once a keyhandler does get 
a lock, it keeps it until it is done.
~Andrew

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