Subj : Re: lockless low-overhead 'pipes' (w/semaphores)
To   : comp.programming.threads
From : Alexander Terekhov
Date : Fri Apr 22 2005 06:34 pm


Chris Thomasson wrote:
> 
> > Apart from the SSE string stuff, on IA32 (compiler reordering aside
> > for a moment), loads have acquire semantics, stores have release
> > semantics, and interlocked stuff is fully-fenced (compound acquire +
> > release semantics).
> 
> Right. The OP should also understand that critical sections can overlap:

Not on IA32 (spinlocks with just release store in unlock aside for a 
moment). Interlocked stuff is fully-fenced on IA32.

> 
> 1: Lock( &l1 );
> 2: l1_dat += 111;
> 3: Unlock( &l1 );
> 
> 4: Lock( &l2 );
> 5: l2_dat += 222;
> 6: Unlock( &l2 );
> 
> Can be legitimately be reordered to:
> 
> 4: Lock( &l2 );
> 5: l2_dat += 222;
> 1: Lock( &l1 );
> 2: l1_dat += 111;
> 6: Unlock( &l2 );
> 3: Unlock( &l1 );

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.

regards,
alexander.

.