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

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.




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