system/linux
system/linux
1
0
Fork 0
mirror of https://github.com/torvalds/linux synced 2024-11-05 18:23:50 +00:00
Code Issues Projects Releases Packages Wiki Activity

tools: memory-model: Document that the LKMM can easily miss control dependencies

Browse source 
Add a small section to the litmus-tests.txt documentation file for
the Linux Kernel Memory Model explaining that the memory model often
fails to recognize certain control dependencies.

Suggested-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Alan Stern 2020-10-03 21:40:22 -04:00 committed by Paul E. McKenney
parent 3650b228f8
commit 9270e1a744
1 changed files with 17 additions and 0 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

17
tools/memory-model/Documentation/litmus-tests.txt
View file

@ -946,6 +946,23 @@ Limitations of the Linux-kernel memory model (LKMM) include:
carrying a dependency, then the compiler can break that dependency
by substituting a constant of that value.
Conversely, LKMM sometimes doesn't recognize that a particular
optimization is not allowed, and as a result, thinks that a
dependency is not present (because the optimization would break it).
The memory model misses some pretty obvious control dependencies
because of this limitation. A simple example is:
r1 = READ_ONCE(x);
if (r1 == 0)
smp_mb();
WRITE_ONCE(y, 1);
There is a control dependency from the READ_ONCE to the WRITE_ONCE,
even when r1 is nonzero, but LKMM doesn't realize this and thinks
that the write may execute before the read if r1 != 0. (Yes, that
doesn't make sense if you think about it, but the memory model's
intelligence is limited.)
2. Multiple access sizes for a single variable are not supported,
and neither are misaligned or partially overlapping accesses.