lwkt_token progress

[Jan Grant]

On Sat, 21 Feb 2004, Matthew Dillon wrote:

> 				Serializing Tokens
>
>     A serializing token may be held by any number of threads simultaniously.
>     A thread holding a token is guarenteed that no other thread also
>     holding that same token will be running at the same time.
>
>     A thread may hold any number of serializing tokens.
>
>     A thread may hold serializing tokens through a thread yield or blocking
>     condition, but must understand that another thread holding those tokens
>     may be allowed to run while the first thread is not running (blocked or
>     yielded away).
>
>     There are theoretically no unresolvable deadlock situations that can
>     arise with the serializing token mechanism.  However, the initial
>     implementation can potentially get into livelock issues with multiply
>     held tokens.
>
>     Serializing tokens may also be used to protect threads from preempting
>     interrupts that attempt to obtain the same token.  This is a slightly
>     different effect from the Big Giant Lock (also known as the MP lock),
>     which does not interlock against interrupts on the same cpu.
>
>     Holding a serializing token does NOT prevent preemptive interrupts
>     from occuring, though it might cause some of those interrupts to
>     block-reschedule.

This seems like a very nice scheme. My initial gut feeling was that it
could potentially lower the error rate for coding against this scheme
compared, say, to the correct application of mutex+lock.

How common are "blocking conditions", though? It occurred to me that
the ease of using serialisation tokens might lead to their prevalence;
have you anything in mind to avoid the situation where a code path
suddenly potentially blocks due to a modification of some function it
(opaquely) depends upon due to a deep call graph?

By "something in mind" I mean some kind of guideline for best use of
these things.

-- 
jan grant, ILRT, University of Bristol. http://www.ilrt.bris.ac.uk/
Tel +44(0)117 9287088 Fax +44 (0)117 9287112 http://ioctl.org/jan/
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