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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.10.2020 16:13, Jürgen Groß wrote:
> 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.

Oh, I did read this latter part as something extra to be aware of,
not as this being the correctness guarantee. Could you make the
connection more clear?

Jan