Re: [PATCH v2 5/8] evtchn: drop acquiring of per-channel lock from send_guest_{global,vcpu}_virq()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 30.10.2020 14:02, Jürgen Groß wrote:
> On 30.10.20 13:52, Jan Beulich wrote:
>> On 30.10.2020 13:27, Jürgen Groß wrote:
>>> On 30.10.20 12:55, Jan Beulich wrote:
>>>> On 30.10.2020 12:15, Jürgen Groß wrote:
>>>>> On 30.10.20 11:57, Julien Grall wrote:
>>>>>> On 30/10/2020 10:49, Jan Beulich wrote:
>>>>>>> On 30.10.2020 11:38, Julien Grall wrote:
>>>>>>>> I think we should consider Juergen's series because the locking for the
>>>>>>>> event channel is easier to understand.
>>>>>>>
>>>>>>> We should, yes. The one thing I'm a little uneasy with is the
>>>>>>> new lock "variant" that gets introduced. Custom locking methods
>>>>>>> also are a common source of problems (which isn't to say I see
>>>>>>> any here).
>>>>>>
>>>>>> I am also unease with a new lock "variant". However, this is the best
>>>>>> proposal I have seen so far to unblock the issue.
>>>>>>
>>>>>> I am open to other suggestion with simple locking discipline.
>>>>>
>>>>> In theory my new lock variant could easily be replaced by a rwlock and
>>>>> using the try-variant for the readers only.
>>>>
>>>> Well, only until we would add check_lock() there, which I think
>>>> we should really have (not just on the slow paths, thanks to
>>>> the use of spin_lock() there). The read-vs-write properties
>>>> you're utilizing aren't applicable in the general case afaict,
>>>> and hence such checking would get in the way.
>>>
>>> No, I don't think so.
>>>
>>> As long as there is no read_lock() user with interrupts off we should be
>>> fine. read_trylock() is no problem as it can't block.
>>
>> How would check_lock() notice the difference? It would be all the
>> same for read and write acquires of the lock, I think, and hence
>> it would still get unhappy about uses from IRQ paths afaict.
> 
> check_lock() isn't applicable here, at least not without modification.
> 
> I think our spinlock implementation is wrong in this regard in case a
> lock is entered via spin_trylock(), BTW. Using spin_trylock() with
> interrupts off for a lock normally taken with interrupts enabled is
> perfectly fine IMO.

Hmm, I think you're right, in which I case guess we ought to relax
this.

Jan