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Re: Interrupt vs Thread - shared resource
- From: Szentirmai Gergely <reg at t-online dot hu>
- To: eCos Discuss <ecos-discuss at ecos dot sourceware dot org>
- Date: Sun, 15 Feb 2009 11:50:42 +0100
- Subject: Re: [ECOS] Interrupt vs Thread - shared resource
- References: <PGEAIGBPLOMOJDAPCOJHIEPMMHAB.email@example.com>
I have read the documentation before, and the main problem is what you
are mentioning too. You cannot call blocking instructions, such as
|cyg_semaphore_wait, so if a thread used the shared resource when the
scheduler started the DSR (we use semaphores) the DSR cannot wait. You
would say we are not allowed to use blocking instructions in DSR
context, but locking the DSR, masking the interrupt, or locking the
scheduler has the same effect, the DSR code is blocked (not at the
theoretical ||cyg_semaphore_wait, ||but at the "start") (or it is an
event worth solution than allow ||cyg_semaphore_wait (rework the
implementation of semphores)).
Paul D. DeRocco Ãrta:
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
Ah, I see your fundamental problem. You've misread the documentation if you think all these functions aren't usable in DSR context. Most are, as long as you don't use a function that may block. Read the bit at the end of each function description in the Reference Manual for details about the valid contexts they can be used in.
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