Re: Mailbox patch

[Nick Garnett <nickg at ecoscentric dot com>]

Nick Garnett <nickg@ecoscentric.com> writes:

> This patch addresses some issues with the mailbox implementation
> raised by the following two threads:
> 
> http://ecos.sourceware.org/ml/ecos-devel/2006-08/msg00004.html
> http://ecos.sourceware.org/ml/ecos-discuss/2006-08/msg00170.html
> 
> The first pointed out a possible race condition with CYG_ASSERTCLASS()
> if it is called without the scheduler lock claimed. The solution has
> been to move all CYG_ASSERTCLASS() calls to places where the scheduler
> lock is claimed.
> 
> 
> The second raised some problems with the use of mailboxes in LWIP. It
> is not clear what the issue there was, I cannot see any obvious
> problems with the mailbox implementation. However, it did highlight
> the fact that the wrong mailbox implementation is being used by
> default.
> 
> There are two implementations, a straightforward one (mboxt) that
> works the same way as most other eCos synchronization primitives, and
> a UITRON-compatible implementation (mboxt2). The main difference is in
> the way that a new message is assigned to a waiting thread, and how
> multiple threads interact (I won't go into gory details here unless
> someone wants me to).
> 
> For some reason the CDL option that causes the mboxt implementation to
> be selected by default was missing, causing the mboxt2 implementation
> to be used all the time. Before looking at this I would have sworn
> that mboxt was the default implementation.
> 
> I have therefore added the option and set it to select the mboxt
> implementation, which is what should have been happening all along. It
> also appears that using mboxt also solves the problems that were
> reported for LWIP.
> 
> However: switching from mboxt2 to mboxt does introduce a slight change
> in semantics for mailboxes. It only affects mailboxes that have
> multiple threads attempting to get messages simultaneously. I believe
> it is reasonable to commit this since it doesn't violate the specified
> behaviour, and it results in using a slightly smaller
> implementation. It has required a change to the test programs, but
> that's because they attempt to track mailbox behaviour in an
> unsynchronized way.
> 
> 
> Before committing this, I think we need to come to a consensus as to
> whether this difference in behaviour is in fact significant.


This has now been committed.



-- 
Nick Garnett                                     eCos Kernel Architect
http://www.ecoscentric.com                The eCos and RedBoot experts
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