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 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.

Jan