RE: Synchronizing Bit operations V2

[Christoph Lameter]

On Thu, 30 Mar 2006, Chen, Kenneth W wrote:

> Christoph Lameter wrote on Thursday, March 30, 2006 7:02 PM
> > We are talking about IA64 and IA64 only generates an single instruction 
> > with either release or acquire semantics for the case in which either 
> > smb_mb__before/after_clear_bit does nothing.
> > 
> > Neither acquire nor release is a memory barrier on IA64.
> 
> 
> The use of
>         smp_mb__before_clear_bit();
>         clear_bit( ... );
> 
> is: all memory operations before this call will be visible before
> the clear_bit().  To me, that's release semantics.

What of it? Release semantics are not a full fence or memory barrier.

> On ia64, we map the following:
> #define Smp_mb__before_clear_bit      do { } while (0)
> #define clear_bit                     clear_bit_mode(..., RELEASE)
> 
> Which looked perfect fine to me.  I don't understand why you say it does
> not provide memory ordering.

It does not provide a memory barrier / fence. Later memory references can 
still be moved by the processor above the instruction with release semantics.
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