thread_info: Add helpers to snapshot thread flags

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@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Marco Elver <elver@google.com>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20211129130653.2037928-2-mark.rutland@arm.com

include/linux/thread_info.h

@@ -118,6 +118,15 @@ 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) \
@@ -130,6 +139,11 @@ 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) \