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Subj : Re: lockless low-overhead 'pipes' (w/semaphores)
To   : comp.programming.threads
From : Alexander Terekhov
Date : Fri Apr 22 2005 07:34 pm


Joe Seigh wrote:
[...]
> > This transformation is not legal. It changes the order of locking
> > operations. In general, it is somewhat hard for compilers (but not
> > hardware) to "overlap" critical regions because lock() can block,
> > but legitimate transformation shall not block preceding unlock.
> >
> Does something somewhere actually say that last bit.  Actually
> they can't overlap because it might cause deadlock, i.e. violate

They can overlap as long as it doesn't cause deadlocks and doesn't
violate acquire-release ordering. In theory, compiler can also 
overlap -- using trylock().

 (assume throw()-nothing)

   lock X
   access A
   unlock X

   lock Y
   access B
   unlock Y

can be tranformed to

   lock X
   b = trylock Y
   if b { access B }
   access A
   unlock X

   if !b { lock, access B }
   unlock Y

regards,
alexander.

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