[ECOS] Interrupt vs Thread - shared resource

Reg reg@t-online.hu
Sat Feb 14 22:32:00 GMT 2009

Thank your for your answer!

I understand, and agree with all the stuff that you wrote (the need of 
ISR, DSR, and the solution). The only reason why I'm calling these 
solutions "hack" (in the meaning, that not the best soulution 
(workaround) ), because the code is depending on the calling 
"enviroment", so the code is somehow application specific.
If for ex. semaphores could be used in DSRs, the code would be better, 
because the implementation of the semaphore would handle this group of 
questions (if you are in a thread, if you are in a DSR etc)
So in other words, when you write a procedure, you have to think about 
"what will call this procedure/ or use the shared resource". If only 
threads use a resource, you use mutex. If a thread and a DSR is using 
it, you have to lock scheduler or mask the interrupt or lock the 
specific DSR, just because no sinchronization object can be used in DSRs.

I think allowing the use of sinchronization objects in DSRs would be a 
great feature from eCos (would need to modify the implemetation of 
semaphore, mutex etc.)
+ better code quality
+ faster developement
+ less bugs...
- a bit more time/resource

So if DSRs are an interface or an intermediate layer between the 
"interrupt world" and the "thread world", why are we not allowed to use 
sinc. objects?

Thank you!
Gergely Szentirmai

Paul D. DeRocco írta:
>> From: Szentirmai Gergely
>> Guys, thank you for your answers!
>> Scheduler lock is an alternative to mask the interrupt, but masking the 
>> specific interrupt (or lock the specific DSR) is maybe better in 
>> performance, but too application specific (code reuse).
>> As I read the scheduler manual:
>> "However there is one situation where locking the scheduler is 
>> appropriate: if some data structure needs to be shared between an 
>> application thread and a DSR associated with some interrupt source, the 
>> thread can use the scheduler lock to prevent concurrent invocations of 
>> the DSR and then safely manipulate the structure."
>> It writes the same thing.
>> So there are two possible solution only; lock the scheduler, or mask the 
>> interrupt.
>> Programatically I think both solution are not "pretty" enough, because I 
>> have to write procedures, that bother with these "hacks" (they are not 
>> high level enough), rather than use a mutex, but this is an eCos 
>> conceptional problem (what a shame, not too user-friendly), I think...
>> Thank you for this dsr_lock idea too!
> I wouldn't call locking the scheduler a hack--it's the normal way to interface between threads and DSRs. If you use semaphores, etc., they do the locking and unlocking for you. Indeed, the whole reason for having DSRs is that it gives you an intermediate layer which can be disabled (by locking the scheduler) without disabling interrupts.
> If you have hardware that needs really fast response time, then you write an ISR which handles the really fast stuff, in which case the DSR has to do hardware-dependent interrupt masking when it accesses resources (buffer pointers, etc.) shared with the ISR.
> If your hardware isn't so insanely fast, then your ISR just invokes the DSR, the DSR talks to the hardware, and the thread "masks" DSRs by locking the scheduler when it accesses resources shared with the DSR.

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