From: Marco Elver <elver@google.com>
Date: Thu, 14 Nov 2019 19:02:59 +0100
Subject: seqlock, kcsan: Add annotations for KCSAN
Patch-mainline: v5.8-rc1
Git-commit: 88ecd153be9519f259b87a9f6f4c8383a8b3bbf1
References: bsc#1176564 bsc#1162702

Since seqlocks in the Linux kernel do not require the use of marked
atomic accesses in critical sections, we teach KCSAN to assume such
accesses are atomic. KCSAN currently also pretends that writes to
`sequence` are atomic, although currently plain writes are used (their
corresponding reads are READ_ONCE).

Further, to avoid false positives in the absence of clear ending of a
seqlock reader critical section (only when using the raw interface),
KCSAN assumes a fixed number of accesses after start of a seqlock
critical section are atomic.

=== Commentary on design around absence of clear begin/end markings ===
Seqlock usage via seqlock_t follows a predictable usage pattern, where
clear critical section begin/end is enforced. With subtle special cases
for readers needing to be flat atomic regions, e.g. because usage such
as in:
  - fs/namespace.c:__legitimize_mnt - unbalanced read_seqretry
  - fs/dcache.c:d_walk - unbalanced need_seqretry

But, anything directly accessing seqcount_t seems to be unpredictable.
Filtering for usage of read_seqcount_retry not following 'do { .. }
while (read_seqcount_retry(..));':

  $ git grep 'read_seqcount_retry' | grep -Ev 'while \(|seqlock.h|Doc|\* '
  => about 1/3 of the total read_seqcount_retry usage.

Just looking at fs/namei.c, we conclude that it is non-trivial to
prescribe and migrate to an interface that would force clear begin/end
seqlock markings for critical sections.

As such, we concluded that the best design currently, is to simply
ensure that KCSAN works well with the existing code.

Signed-off-by: Marco Elver <elver@google.com>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
[dwagner: undone kcsan changes, just keeping READ_ONCE in
  __read_seqcount_retry()]
Acked-by: Daniel Wagner <dwagner@suse.de>
---
 include/linux/seqlock.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -202,7 +202,7 @@ static inline unsigned raw_seqcount_begi
  */
 static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
 {
-	return unlikely(s->sequence != start);
+	return unlikely(READ_ONCE(s->sequence) != start);
 }
 
 /**