On Fri, Nov 06, 2020 at 03:40:08PM -0500, Alan Stern wrote: > On Fri, Nov 06, 2020 at 11:59:12AM -0800, Paul E. McKenney wrote: > > On Fri, Nov 06, 2020 at 02:23:51PM -0500, Alan Stern wrote: > > > On Fri, Nov 06, 2020 at 10:04:46AM -0800, Paul E. McKenney wrote: > > > > On Fri, Nov 06, 2020 at 11:59:30AM -0500, Alan Stern wrote: > > > > > > + See also "Control Dependency". > > > > > > > > > > There should also be an entry for "Data Dependency", linked from here > > > > > and from Control Dependency. > > > > > > > > > > > +Marked Access: An access to a variable that uses an special function or > > > > > > + macro such as "r1 = READ_ONCE()" or "smp_store_release(&a, 1)". > > > > > > > > > > How about "r1 = READ_ONCE(x)"? > > > > > > > > Good catches! I am planning to squash the commit below into the > > > > original. Does that cover it? > > > > > > No, because you didn't add a glossary entry for "Data Dependency" and > > > there's no link from "Control Dependency" to "Data Dependency". > > > > Sigh. I was thinking "entry in the list", and didn't even thing to > > check for an entry in the glossary as a whole. With the patch below > > (on top of the one sent earlier), are we good? > > > > Thanx, Paul > > > > ------------------------------------------------------------------------ > > > > commit 5a49c32551e83d30e304d6c3fbb660737ba2654e > > Author: Paul E. McKenney <paulmck@xxxxxxxxxx> > > Date: Fri Nov 6 11:57:25 2020 -0800 > > > > fixup! tools/memory-model: Add a glossary of LKMM terms > > > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > > > > diff --git a/tools/memory-model/Documentation/glossary.txt b/tools/memory-model/Documentation/glossary.txt > > index 471bf13..b2da636 100644 > > --- a/tools/memory-model/Documentation/glossary.txt > > +++ b/tools/memory-model/Documentation/glossary.txt > > @@ -64,7 +64,7 @@ Control Dependency: When a later store's execution depends on a test > > fragile, and can be easily destroyed by optimizing compilers. > > Please see control-dependencies.txt for more information. > > > > - See also "Address Dependency". > > + See also "Address Dependency" and "Data Dependency". > > > > Cycle: Memory-barrier pairing is restricted to a pair of CPUs, as the > > name suggests. And in a great many cases, a pair of CPUs is all > > @@ -85,6 +85,23 @@ Cycle: Memory-barrier pairing is restricted to a pair of CPUs, as the > > > > See also "Pairing". > > > > +Data Dependency: When the data written by a later store is computed based > > + on the value returned by an earlier load, a "data dependency" > > + extends from that load to that later store. For example: > > + > > + 1 r1 = READ_ONCE(x); > > + 2 WRITE_ONCE(y, r1 + 1); > > + > > + In this case, the data dependency extends from the READ_ONCE() > > + on line 1 to the WRITE_ONCE() on line 2. Data dependencies are > > + fragile and can be easily destroyed by optimizing compilers. > > + Because optimizing compilers put a great deal of effort into > > + working out what values integer variables might have, this is > > + especially true in cases where the dependency is carried through > > + an integer. > > + > > + See also "Address Dependency" and "Control Dependency". > > + > > From-Reads (fr): When one CPU's store to a given variable happened > > too late to affect the value returned by another CPU's > > load from that same variable, there is said to be a from-reads > > Yes, this is better. Thank you for bearing with me on this! > Is it really true that data dependencies are so easily destroyed? I > would expect that a true "semantic" dependency (i.e., one where the > value written really does vary according to the value read) would be > rather hard to second guess. The usual optimizations apply, for but one example: r1 = READ_ONCE(x); WRITE_ONCE(y, (r1 + 1) % MAX_ELEMENTS); If MAX_ELEMENTS is 1, so long, data dependency! With pointers, the compiler has fewer optimization opportunities, but there are still cases where it can break the dependency. Or transform it to a control dependency. Thanx, Paul
