[Click] write handler synchronization

[Eddie Kohler]

Roman Chertov wrote:
> Hi Eddie,
> 
>> Normally run_task will NOT execute in parallel with a write handler.  
>> Write handlers are by default "exclusive," meaning that all threads 
>> are paused before the write handler executes.
> 
> Does this mean that if I have say 4 threads in kernel level then all 4 
> would be blocked until a call to a write handler is made through the 
> click fs?

Yes, all threads are blocked while the write handelr is called.

> I am creating an element that takes a scenario file for a 
> link and adds loss and bit corruption.  I need the user app to 
> periodically generate and load new events into the Click element via 
> clickfs.  There are at least two event vectors with one being available 
> to be populated with events through clickfs, and the other is used by 
> the Click element itself.  When the Click element exhausts all the 
> events in the current vector it then swaps the vectors, so that the just 
> exhausted vector becomes available for writing to by the user app.  So I 
> am concerned about synchronization (which seems to be not an issue). But 
> this raises a problem of all of the tasks blocking for the duration of 
> the update.  Is there a way to avoid this?  I wouldn't mind writing the 
> synchronization code to avoid the potentially lengthy block.

Yes.  Simply mark the handler as NONEXCLUSIVE using the Handler::NONEXCLUSIVE 
flag.  Then you'll need to handle synchronization, but the handler won't block.

Eddie


> 
> Thanks,
> 
> Roman
> 
>>
>> Note that this ONLY applies to write handlers accessed via the Click 
>> file system.  Write handlers called by, for example, Script elements 
>> are not subject to the same constraints.  (Maybe this should be 
>> changed?...)
>>
>> Eddie
>>
>>
>> Roman Chertov wrote:
>>> Hello,
>>>     I am making an element which will vary link properties based on a 
>>> provided scenario file.  A user app will periodically communicate 
>>> with the Click module via the read/write handlers.  Is it possible 
>>> for run_task to execute while a write handler is executing?  This can 
>>> potentially corrupt the data structure which houses the link events.  
>>> So I am curious if I need to add synchronization or not.
>>>
>>> Thanks,
>>>
>>> Roman
>>> _______________________________________________
>>> click mailing list
>>> click at amsterdam.lcs.mit.edu
>>> https://amsterdam.lcs.mit.edu/mailman/listinfo/click
>>
>