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

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.


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