The following commit has been merged into the locking/core branch of tip:
Commit-ID: 183ec5f26b2fc97a4a9871865bfe9b33c41fddb2
Gitweb: https://git.kernel.org/tip/183ec5f26b2fc97a4a9871865bfe9b33c41fddb2
Author: Marco Elver <elver@xxxxxxxxxx>
AuthorDate: Mon, 04 Nov 2024 16:43:09 +01:00
Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 05 Nov 2024 12:55:35 +01:00
kcsan, seqlock: Fix incorrect assumption in read_seqbegin()
During testing of the preceding changes, I noticed that in some cases,
current->kcsan_ctx.in_flat_atomic remained true until task exit. This is
obviously wrong, because _all_ accesses for the given task will be
treated as atomic, resulting in false negatives i.e. missed data races.
Debugging led to fs/dcache.c, where we can see this usage of seqlock:
struct dentry *d_lookup(const struct dentry *parent, const struct qstr *name)
{
struct dentry *dentry;
unsigned seq;
do {
seq = read_seqbegin(&rename_lock);
dentry = __d_lookup(parent, name);
if (dentry)
break;
} while (read_seqretry(&rename_lock, seq));
[...]
As can be seen, read_seqretry() is never called if dentry != NULL;
consequently, current->kcsan_ctx.in_flat_atomic will never be reset to
false by read_seqretry().
Give up on the wrong assumption of "assume closing read_seqretry()", and
rely on the already-present annotations in read_seqcount_begin/retry().
Fixes: 88ecd153be95 ("seqlock, kcsan: Add annotations for KCSAN")
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20241104161910.780003-6-elver@xxxxxxxxxx
---
include/linux/seqlock.h | 12 +-----------
1 file changed, 1 insertion(+), 11 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 45eee0e..5298765 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -810,11 +810,7 @@ static __always_inline void write_seqcount_latch_end(seqcount_latch_t *s)
*/
static inline unsigned read_seqbegin(const seqlock_t *sl)
{
- unsigned ret = read_seqcount_begin(&sl->seqcount);
-
- kcsan_atomic_next(0); /* non-raw usage, assume closing read_seqretry() */
- kcsan_flat_atomic_begin();
- return ret;
+ return read_seqcount_begin(&sl->seqcount);
}
/**
@@ -830,12 +826,6 @@ static inline unsigned read_seqbegin(const seqlock_t *sl)
*/
static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
{
- /*
- * Assume not nested: read_seqretry() may be called multiple times when
- * completing read critical section.
- */
- kcsan_flat_atomic_end();
-
return read_seqcount_retry(&sl->seqcount, start);
}
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