Merge tag 'kcsan.2023.04.04a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu

Pull KCSAN updates from Paul McKenney:
 "Kernel concurrency sanitizer (KCSAN) updates for v6.4

  This fixes kernel-doc warnings and also updates instrumentation from
  READ_ONCE() to volatile in order to avoid unaligned load-acquire
  instructions on arm64 in kernels built with LTO"

* tag 'kcsan.2023.04.04a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu:
  kcsan: Avoid READ_ONCE() in read_instrumented_memory()
  instrumented.h: Fix all kernel-doc format warnings

1 files changed, 13 insertions, 4 deletions

diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 54d077e1a2dc..5a60cc52adc0 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -337,11 +337,20 @@ static void delay_access(int type)
  */
 static __always_inline u64 read_instrumented_memory(const volatile void *ptr, size_t size)
 {
+	/*
+	 * In the below we don't necessarily need the read of the location to
+	 * be atomic, and we don't use READ_ONCE(), since all we need for race
+	 * detection is to observe 2 different values.
+	 *
+	 * Furthermore, on certain architectures (such as arm64), READ_ONCE()
+	 * may turn into more complex instructions than a plain load that cannot
+	 * do unaligned accesses.
+	 */
 	switch (size) {
-	case 1:  return READ_ONCE(*(const u8 *)ptr);
-	case 2:  return READ_ONCE(*(const u16 *)ptr);
-	case 4:  return READ_ONCE(*(const u32 *)ptr);
-	case 8:  return READ_ONCE(*(const u64 *)ptr);
+	case 1:  return *(const volatile u8 *)ptr;
+	case 2:  return *(const volatile u16 *)ptr;
+	case 4:  return *(const volatile u32 *)ptr;
+	case 8:  return *(const volatile u64 *)ptr;
 	default: return 0; /* Ignore; we do not diff the values. */
 	}
 }