From: paulmck@kernel.org
To: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
kernel-team@fb.com, mingo@kernel.org
Cc: stern@rowland.harvard.edu, parri.andrea@gmail.com,
will@kernel.org, peterz@infradead.org, boqun.feng@gmail.com,
npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk,
luc.maranget@inria.fr, akiyks@gmail.com,
"Paul E . McKenney" <paulmck@kernel.org>
Subject: [PATCH memory-model 1/8] tools: memory-model: Document that the LKMM can easily miss control dependencies
Date: Thu, 5 Nov 2020 14:00:10 -0800 [thread overview]
Message-ID: <20201105220017.15410-1-paulmck@kernel.org> (raw)
In-Reply-To: <20201105215953.GA15309@paulmck-ThinkPad-P72>
From: Alan Stern <stern@rowland.harvard.edu>
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>
---
tools/memory-model/Documentation/litmus-tests.txt | 17 +++++++++++++++++
1 file changed, 17 insertions(+)
diff --git a/tools/memory-model/Documentation/litmus-tests.txt b/tools/memory-model/Documentation/litmus-tests.txt
index 2f840dc..8a9d5d2 100644
--- a/tools/memory-model/Documentation/litmus-tests.txt
+++ b/tools/memory-model/Documentation/litmus-tests.txt
@@ -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.
--
2.9.5
next prev parent reply other threads:[~2020-11-05 22:00 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-05 21:59 [PATCH memory-model 0/8] LKMM updates for v5.11 Paul E. McKenney
2020-11-05 22:00 ` paulmck [this message]
2020-11-05 22:00 ` [PATCH memory-model 2/8] tools/memory-model: Move Documentation description to Documentation/README paulmck
2020-11-05 22:00 ` [PATCH memory-model 3/8] tools/memory-model: Document categories of ordering primitives paulmck
2020-11-06 16:56 ` Alan Stern
2020-11-06 19:11 ` Paul E. McKenney
2020-11-05 22:00 ` [PATCH memory-model 4/8] docs/memory-barriers.txt: Fix a typo in CPU MEMORY BARRIERS section paulmck
2020-11-05 22:00 ` [PATCH memory-model 5/8] tools/memory-model: Add a glossary of LKMM terms paulmck
2020-11-06 1:47 ` Boqun Feng
2020-11-06 18:01 ` Paul E. McKenney
2020-11-07 3:07 ` Boqun Feng
2020-11-06 16:59 ` Alan Stern
2020-11-06 18:04 ` Paul E. McKenney
2020-11-06 19:23 ` Alan Stern
2020-11-06 19:59 ` Paul E. McKenney
2020-11-06 20:40 ` Alan Stern
2020-11-06 21:04 ` Paul E. McKenney
2020-11-07 2:32 ` Alan Stern
2020-11-05 22:00 ` [PATCH memory-model 6/8] tools/memory-model: Add types to litmus tests paulmck
2020-11-05 22:41 ` Akira Yokosawa
2020-11-05 22:56 ` Paul E. McKenney
2020-11-25 11:34 ` Akira Yokosawa
2020-11-27 15:46 ` Paul E. McKenney
2020-11-28 5:56 ` Akira Yokosawa
2020-11-28 6:00 ` [PATCH 1/2] tools/memory-model: Remove redundant initialization in " Akira Yokosawa
2020-11-28 6:01 ` [PATCH 2/2] tools/memory-model: Fix typo in klitmus7 compatibility table Akira Yokosawa
2020-11-29 3:33 ` Paul E. McKenney
2020-11-05 22:00 ` [PATCH memory-model 7/8] tools/memory-model: Use "buf" and "flag" for message-passing tests paulmck
2020-11-05 22:00 ` [PATCH memory-model 8/8] tools/memory-model: Label MP tests' producers and consumers paulmck
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20201105220017.15410-1-paulmck@kernel.org \
--to=paulmck@kernel.org \
--cc=akiyks@gmail.com \
--cc=boqun.feng@gmail.com \
--cc=dhowells@redhat.com \
--cc=j.alglave@ucl.ac.uk \
--cc=kernel-team@fb.com \
--cc=linux-arch@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=luc.maranget@inria.fr \
--cc=mingo@kernel.org \
--cc=npiggin@gmail.com \
--cc=parri.andrea@gmail.com \
--cc=peterz@infradead.org \
--cc=stern@rowland.harvard.edu \
--cc=will@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).