6.12-stable review patch. If anyone has any objections, please let me know.
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From: Marco Elver elver@google.com
[ Upstream commit 183ec5f26b2fc97a4a9871865bfe9b33c41fddb2 ]
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@google.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Link: https://lore.kernel.org/r/20241104161910.780003-6-elver@google.com Signed-off-by: Sasha Levin sashal@kernel.org --- 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 45eee0e5dca01..5298765d6ca48 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); }