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

["Jürgen Groß" <jgross@suse.com>]

On 30.10.20 14:38, Jan Beulich wrote:
> 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.

Just writing a patch. :-)

And one for adding check_lock() to rwlocks, too.


Juergen