sched: Snapshot thread flags

Some thread flags can be set remotely, and so even when IRQs are disabled,
the flags can change under our feet. Generally this is unlikely to cause a
problem in practice, but it is somewhat unsound, and KCSAN will
legitimately warn that there is a data race.

To avoid such issues, a snapshot of the flags has to be taken prior to
using them. Some places already use READ_ONCE() for that, others do not.

Convert them all to the new flag accessor helpers.

The READ_ONCE(ti->flags) .. cmpxchg(ti->flags) loop in
set_nr_if_polling() is left as-is for clarity.

Signed-off-by: Mark Rutland <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Acked-by: Paul E. McKenney <[email protected]>
Cc: Juri Lelli <[email protected]>
Cc: Vincent Guittot <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: mrutland-arm

kernel/sched/core.c

@@ -8520,7 +8520,7 @@ void sched_show_task(struct task_struct *p)
 	rcu_read_unlock();
 	pr_cont(" stack:%5lu pid:%5d ppid:%6d flags:0x%08lx\n",
 		free, task_pid_nr(p), ppid,
-		(unsigned long)task_thread_info(p)->flags);
+		read_task_thread_flags(p));
 
 	print_worker_info(KERN_INFO, p);
 	print_stop_info(KERN_INFO, p);