Re: [PATCH v2 2/2] xen/evtchn: rework per event channel lock

[Jürgen Groß <jgross@xxxxxxxx>]

On 13.10.20 16:02, Jan Beulich wrote:
> On 12.10.2020 11:27, 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
> And how come omitting the send or returning default state is valid?
This is explained in the part of the commit message you didn't cite:

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.


Juergen