Re: [PATCH v6 2/3] xen/evtchn: rework per event channel lock

[Julien Grall <julien@xxxxxxx>]

Hi Juergen,

On 09/11/2020 16:38, 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 event channel state
> changes (creation, closing, binding) against normal operations (set
> pending, [un]masking, priority changes).
>
> Use a rwlock, but with some restrictions:
>
> - Changing the state of an event channel (creation, closing, binding)
>    needs to use write_lock(), with ASSERT()ing that the lock is taken as
>    writer only when the state of the event channel is either before or
>    after the locked region appropriate (either free or unbound).
>
> - Sending an event needs to use read_trylock() mostly, in case of not
>    obtaining the lock the operation is omitted. This is needed as
>    sending an event can happen with interrupts off (at least in some
>    cases).
>
> - Dumping the event channel state for debug purposes is using
>    read_trylock(), too, in order to avoid blocking in case the lock is
>    taken as writer for a long time.
>
> - All other cases can use read_lock().
>
> Fixes: e045199c7c9c54 ("evtchn: address races with evtchn_reset()")
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Acked-by: Julien Grall <jgrall@xxxxxxxxxx>

Cheers,

--
Julien Grall