Re: [PATCH 2/2] xen/evtchn: rework per event channel lock

[Jürgen Groß <jgross@xxxxxxxx>]

On 12.10.20 11:10, Juergen Gross wrote:
> Currently the lock for a single event channel needs to be taken with
> interrupts off, which causes deadlocks in some cases.
>
> Rework the per event channel lock to be non-blocking for the case of
> sending an event and removing the need for disabling interrupts for
> taking the lock.
>
> The lock is needed for avoiding races between sending an event or
> querying the channel's state against removal of the event channel.
>
> Use a locking scheme similar to a rwlock, but with some modifications:
>
> - sending an event or querying the event channel's state uses an
>    operation similar to read_trylock(), in case of not obtaining the
>    lock the sending is omitted or a default state is returned
>
> - closing an event channel is similar to write_lock(), but without
>    real fairness regarding multiple writers (this saves some space in
>    the event channel structure and multiple writers are impossible as
>    closing an event channel requires the domain's event_lock to be
>    held).
>
> With this locking scheme it is mandatory that a writer will always
> either start with an unbound or free event channel or will end with
> an unbound or free event channel, as otherwise the reaction of a reader
> not getting the lock would be wrong.
>
> Fixes: e045199c7c9c54 ("evtchn: address races with evtchn_reset()")
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Oh, please ignore. I forgot to add the needed barriers to the locking
functions.


Juergen