[PATCH v2 1/9] thread_info: add helpers to snapshot thread flags

[Mark Rutland]

In <linux/thread_info.h> there are helpers to manipulate individual
thread flags, but where code wants to check several flags at once, it
must open code reading current_thread_info()->flags and operating on a
snapshot.

As some flags can be set remotely it's necessary to use READ_ONCE() to
get a consistent snapshot even when IRQs are disabled, but some code
forgets to do this. Generally this is unlike to cause a problem in
practice, but it is somewhat unsound, and KCSAN will legitimately warn
that there is a data race.

To make it easier to do the right thing, and to highlight that
concurrent modification is possible, add new helpers to snapshot the
flags, which should be used in preference to plain reads. Subsequent
patches will move existing code to use the new helpers.

Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Marco Elver <elver@xxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
---
 include/linux/thread_info.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
index 157762db9d4b..a58e204adcc5 100644
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h
@@ -117,6 +117,15 @@ static inline int test_ti_thread_flag(struct thread_info *ti, int flag)
 	return test_bit(flag, (unsigned long *)&ti->flags);
 }
 
+/*
+ * This may be used in noinstr code, and needs to be __always_inline to prevent
+ * inadvertent instrumentation.
+ */
+static __always_inline unsigned long read_ti_thread_flags(struct thread_info *ti)
+{
+	return READ_ONCE(ti->flags);
+}
+
 #define set_thread_flag(flag) \
 	set_ti_thread_flag(current_thread_info(), flag)
 #define clear_thread_flag(flag) \
@@ -129,6 +138,11 @@ static inline int test_ti_thread_flag(struct thread_info *ti, int flag)
 	test_and_clear_ti_thread_flag(current_thread_info(), flag)
 #define test_thread_flag(flag) \
 	test_ti_thread_flag(current_thread_info(), flag)
+#define read_thread_flags() \
+	read_ti_thread_flags(current_thread_info())
+
+#define read_task_thread_flags(t) \
+	read_ti_thread_flags(task_thread_info(t))
 
 #ifdef CONFIG_GENERIC_ENTRY
 #define set_syscall_work(fl) \
-- 
2.11.0