On Tue, Feb 17, 2015 at 07:23:35PM +0100, Peter Zijlstra wrote:
> On Tue, Feb 17, 2015 at 10:01:54AM -0800, Paul E. McKenney wrote:
> > > > The blob under SMP BARRIER PAIRING does not mention pairing with control
> > > > dependencies; and I'm rather sure I've done so.
> >
> > And here is a patch for the control-dependency pairing. Thoughts?
>
> The proposed patch does not change the blub under SMP BARRIER PAIRING,
> which would be the first place I would look for this.
OK, updated below.
> > @@ -850,6 +853,19 @@ Or:
> > <data dependency barrier>
> > y = *x;
> >
> > +Or even:
> > +
> > + CPU 1 CPU 2
> > + =============== ===============================
> > + r1 = ACCESS_ONCE(y);
> > + <write barrier>
> > + ACCESS_ONCE(y) = 1; if (r2 = ACCESS_ONCE(x)) {
> > + <implicit control dependency>
> > + ACCESS_ONCE(y) = 1;
> > + }
> > +
> > + assert(r1 == 0 || r2 == 0);
> > +
> > Basically, the read barrier always has to be there, even though it can be
> > of
> > the "weaker" type.
>
> Does that want to be a <general barrier>; CPU1 looks to do a LOAD
> followed by a STORE, separated by a WMB, which is of course odd.
>
> To me the pairing with a general barrier is also the most intuitive,
> since the control dependency is a LOAD -> STORE order.
>
> Then again, I'm rather tired and maybe I'm missing the obvious :-)
Nope, you are correct. There either must be a general barrier or
CPU 1's write must be an smp_store_release(). I went with the
general barrier. Please see below for an update.
Thanx, Paul
------------------------------------------------------------------------
documentation: Clarify control-dependency pairing
This commit explicitly states that control dependencies pair normally
with other barriers, and gives an example of such pairing.
Reported-by: Peter Zijlstra <pet...@infradead.org>
Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
diff --git a/Documentation/memory-barriers.txt
b/Documentation/memory-barriers.txt
index ca2387ef27ab..6974f1c2b4e1 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -592,9 +592,9 @@ See also the subsection on "Cache Coherency" for a more
thorough example.
CONTROL DEPENDENCIES
--------------------
-A control dependency requires a full read memory barrier, not simply a data
-dependency barrier to make it work correctly. Consider the following bit of
-code:
+A load-load control dependency requires a full read memory barrier, not
+simply a data dependency barrier to make it work correctly. Consider the
+following bit of code:
q = ACCESS_ONCE(a);
if (q) {
@@ -615,14 +615,15 @@ case what's actually required is:
}
However, stores are not speculated. This means that ordering -is- provided
-in the following example:
+for load-store control dependencies, as in the following example:
q = ACCESS_ONCE(a);
if (q) {
ACCESS_ONCE(b) = p;
}
-Please note that ACCESS_ONCE() is not optional! Without the
+Control dependencies pair normally with other types of barriers.
+That said, please note that ACCESS_ONCE() is not optional! Without the
ACCESS_ONCE(), might combine the load from 'a' with other loads from
'a', and the store to 'b' with other stores to 'b', with possible highly
counterintuitive effects on ordering.
@@ -813,6 +814,8 @@ In summary:
barrier() can help to preserve your control dependency. Please
see the Compiler Barrier section for more information.
+ (*) Control dependencies pair normally with other types of barriers.
+
(*) Control dependencies do -not- provide transitivity. If you
need transitivity, use smp_mb().
@@ -823,14 +826,14 @@ SMP BARRIER PAIRING
When dealing with CPU-CPU interactions, certain types of memory barrier should
always be paired. A lack of appropriate pairing is almost certainly an error.
-General barriers pair with each other, though they also pair with
-most other types of barriers, albeit without transitivity. An acquire
-barrier pairs with a release barrier, but both may also pair with other
-barriers, including of course general barriers. A write barrier pairs
-with a data dependency barrier, an acquire barrier, a release barrier,
-a read barrier, or a general barrier. Similarly a read barrier or a
-data dependency barrier pairs with a write barrier, an acquire barrier,
-a release barrier, or a general barrier:
+General barriers pair with each other, though they also pair with most
+other types of barriers, albeit without transitivity. An acquire barrier
+pairs with a release barrier, but both may also pair with other barriers,
+including of course general barriers. A write barrier pairs with a data
+dependency barrier, a control dependency, an acquire barrier, a release
+barrier, a read barrier, or a general barrier. Similarly a read barrier,
+control dependency, or a data dependency barrier pairs with a write
+barrier, an acquire barrier, a release barrier, or a general barrier:
CPU 1 CPU 2
=============== ===============
@@ -850,6 +853,19 @@ Or:
<data dependency barrier>
y = *x;
+Or even:
+
+ CPU 1 CPU 2
+ =============== ===============================
+ r1 = ACCESS_ONCE(y);
+ <general barrier>
+ ACCESS_ONCE(y) = 1; if (r2 = ACCESS_ONCE(x)) {
+ <implicit control dependency>
+ ACCESS_ONCE(y) = 1;
+ }
+
+ assert(r1 == 0 || r2 == 0);
+
Basically, the read barrier always has to be there, even though it can be of
the "weaker" type.
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