linux-kernel - Re: Uses for memory barriers

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Date:	Sun, 22 Oct 2006 22:32:23 -0700
From:	"Paul E. McKenney" <paulmck@...ibm.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	David Howells <dhowells@...hat.com>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Uses for memory barriers

On Sat, Oct 21, 2006 at 10:18:38PM -0400, Alan Stern wrote:
> On Sat, 21 Oct 2006, Paul E. McKenney wrote:
> 
> > > This is identical to the previous version, since by definition
> > > 
> > > 	st_i(B) ==> ld_j(B)  is equivalent to  st_i(B) => ld_j(B) &&
> > > 		not exist k: st_i(B) => st_k(B) => ld_j(B).
> > 
> > OK -- we were assuming slightly different definitions of "==>".  I as
> > assuming that if st==>ld1==>ld2, that it is not the case that "st==>ld2".
> > In this circumstance, your definition is certainly more convenient than
> > is mine.  In the case of MMIO, the situation might be reversed.
> 
> MMIO of course is completely different.  For regular memory accesses I 
> think we should never allow a load on the left side of "=>" or "==>".  
> Keep them invisible!  :-)
> 
> Writing ld(A) => st(A) is bad because (1) it suggests that the store
> somehow "sees" the load (which it doesn't; the load is invisible), and (2)  
> it suggests that the store occurs "later" in some sense than the load
> (which might not be true, since a load doesn't necessarily return the
> value of the temporally most recent store).

How about ld_i(A) => ld_j(A)?  This would say that both loads corresponded
to the same store.

> My viewpoint is that "=>" really provides an ordering of stores only.  
> Its use with loads is something of an artifact; it gives a convenient way
> of expressing the fact that a load "sees" an initial segment of all the
> stores to a variable (and the value it returns is that of the last store
> in the segment).

Seems reasonable at first glance, give or take comparing two loads.

> > > (2) doesn't make sense, since loads aren't part of the global ordering of
> > > accesses of B -- they are invisible.  (BTW, you don't need to assume as
> > > well that stores are blind; it's enough just to have loads be invisible.)  
> > > Each load sees an initial sequence of stores ending in the store whose
> > > value is returned by the load, but this doesn't mean that the load occurs
> > > between that store and the next one.
> > 
> > That is due to our difference in definition.  Perhaps the following
> > definition:  "A==>B" means either that B sees the value stored by A
> > or that B sees the same value as does A?
> > 
> > Some work will be required to see what is best.
> 
> How about this instead: "A==>B" means that B sees the value stored by A,
> and "A==B" means that A and B are both loads and they see the value from
> the same store.  That way we avoid putting a load on the left side of
> "==>".

My concern is that "==" might also have connotations of equal values from
distinct stores.

						Thanx, Paul
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