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



Hi,

On 09/11/2020 16:48, Jan Beulich wrote:
On 09.11.2020 17:38, 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>
---
V4:
- switch to rwlock
- add ASSERT() to verify correct write_lock() usage

V3:
- corrected a copy-and-paste error (Jan Beulich)
- corrected unlocking in two cases (Jan Beulich)
- renamed evtchn_read_trylock() (Jan Beulich)
- added some comments and an ASSERT() for evtchn_write_lock()
- set EVENT_WRITE_LOCK_INC to INT_MIN

V2:
- added needed barriers

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

I'll give it overnight for others to possibly comment, but I'd
like to get this in tomorrow if at all possible.

IIRC, Citrix originally reported the issues. I would like to give them an opportunity to test the patches and confirm this effectively fixed there issues.

@Roger, @Andrew, could you give a try to confirm this unblock the vm event issue?

Cheers,

--
Julien Grall



 


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