kcsan: Add current->state to implicitly atomic accesses

Add volatile current->state to list of implicitly atomic accesses. This
is in preparation to eventually enable KCSAN on kernel/sched (which
currently still has KCSAN_SANITIZE := n).

Since accesses that match the special check in atomic.h are rare, it
makes more sense to move this check to the slow-path, avoiding the
additional compare in the fast-path. With the microbenchmark, a speedup
of ~6% is measured.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

1 files changed, 15 insertions, 7 deletions

diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 065615df88ea..eb30ecdc8c00 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -188,12 +188,13 @@ static __always_inline struct kcsan_ctx *get_ctx(void)
 	return in_task() ? &current->kcsan_ctx : raw_cpu_ptr(&kcsan_cpu_ctx);
 }
 
+/* Rules for generic atomic accesses. Called from fast-path. */
 static __always_inline bool
 is_atomic(const volatile void *ptr, size_t size, int type)
 {
 	struct kcsan_ctx *ctx;
 
-	if ((type & KCSAN_ACCESS_ATOMIC) != 0)
+	if (type & KCSAN_ACCESS_ATOMIC)
 		return true;
 
 	/*
@@ -201,16 +202,16 @@ is_atomic(const volatile void *ptr, size_t size, int type)
 	 * as atomic. This allows using them also in atomic regions, such as
 	 * seqlocks, without implicitly changing their semantics.
 	 */
-	if ((type & KCSAN_ACCESS_ASSERT) != 0)
+	if (type & KCSAN_ACCESS_ASSERT)
 		return false;
 
 	if (IS_ENABLED(CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC) &&
-	    (type & KCSAN_ACCESS_WRITE) != 0 && size <= sizeof(long) &&
+	    (type & KCSAN_ACCESS_WRITE) && size <= sizeof(long) &&
 	    IS_ALIGNED((unsigned long)ptr, size))
 		return true; /* Assume aligned writes up to word size are atomic. */
 
 	ctx = get_ctx();
-	if (unlikely(ctx->atomic_next > 0)) {
+	if (ctx->atomic_next > 0) {
 		/*
 		 * Because we do not have separate contexts for nested
 		 * interrupts, in case atomic_next is set, we simply assume that
@@ -224,10 +225,8 @@ is_atomic(const volatile void *ptr, size_t size, int type)
 			--ctx->atomic_next; /* in task, or outer interrupt */
 		return true;
 	}
-	if (unlikely(ctx->atomic_nest_count > 0 || ctx->in_flat_atomic))
-		return true;
 
-	return kcsan_is_atomic(ptr);
+	return ctx->atomic_nest_count > 0 || ctx->in_flat_atomic;
 }
 
 static __always_inline bool
@@ -367,6 +366,15 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 	if (!kcsan_is_enabled())
 		goto out;
 
+	/*
+	 * Special atomic rules: unlikely to be true, so we check them here in
+	 * the slow-path, and not in the fast-path in is_atomic(). Call after
+	 * kcsan_is_enabled(), as we may access memory that is not yet
+	 * initialized during early boot.
+	 */
+	if (!is_assert && kcsan_is_atomic_special(ptr))
+		goto out;
+
 	if (!check_encodable((unsigned long)ptr, size)) {
 		kcsan_counter_inc(KCSAN_COUNTER_UNENCODABLE_ACCESSES);
 		goto out;