Subj : CAS question
To   : comp.programming.threads
From : Toby Douglass
Date : Fri Mar 25 2005 07:51 pm

I'm currently thinking about lockless locking.

One of the fundamental operations used is Compare-And-Swap.

The Windows API for this is;

LONG InterlockedCompareExchange( LONG volatile *Destination, LONG 
Exchange, LONG Comperand );

So for example, one might loop like this;

while( my_ticket != InterlockedCompareExchange(current_ticket, 
my_ticket, my_ticket) );

My question is to do with the arguments to the CAS function call.

I had thought that it would be invalid to perform any in-line operations 
upon the arguments to the function, since that would have to be done 
before or after the actual atomic instruction was executed in the 
machine code, and so would not be atomic.

So for example, this;

while( count+1 != InterlockedCompareExchange(&counter, count+1, 
my_count) );

would be invalid, since the "counter+1" would in fact occur *before* the 
CAS instruction.

However, I've just seen this page;

http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/queues.ht
ml

Which seems to imply this in-line work *can* be done.

My question is: is this so?  can I perform any in-line operations on the 
arguments to CAS?

-- 
Toby

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