[SRU Hirsute 1/1] sched: Stop PF_NO_SETAFFINITY from being inherited by various init system threads

Thadeu Lima de Souza Cascardo cascardo at canonical.com
Mon Aug 16 17:40:33 UTC 2021

From: Frederic Weisbecker <frederic at kernel.org>

BugLink: https://bugs.launchpad.net/bugs/1940107


  00b89fe0197f ("sched: Make the idle task quack like a per-CPU kthread")

... added PF_KTHREAD | PF_NO_SETAFFINITY to the idle kernel threads.

Unfortunately these properties are inherited to the init/0 children
through kernel_thread() calls: init/1 and kthreadd. There are several
side effects to that:

1) kthreadd affinity can not be reset anymore from userspace. Also
   PF_NO_SETAFFINITY propagates to all kthreadd children, including
   the unbound kthreads Therefore it's not possible anymore to overwrite
   the affinity of any of them. Here is an example of warning reported
   by rcutorture:

		WARNING: CPU: 0 PID: 116 at kernel/rcu/tree_nocb.h:1306 rcu_bind_current_to_nocb+0x31/0x40
		Call Trace:

2) init/1 does an exec() in the end which clears both
   PF_KTHREAD and PF_NO_SETAFFINITY so we are fine once kernel_init()
   escapes to userspace. But until then, no initcall or init code can
   successfully call sched_setaffinity() to init/1.

   Also PF_KTHREAD looks legit on init/1 before it calls exec() but
   we better be careful with unknown introduced side effects.

One way to solve the PF_NO_SETAFFINITY issue is to not inherit this flag
on copy_process() at all. The cases where it matters are:

* fork_idle(): explicitly set the flag already.
* fork() syscalls: userspace tasks that shouldn't be concerned by that.
* create_io_thread(): the callers explicitly attribute the flag to the
                      newly created tasks.
* kernel_thread():
	- Fix the issues on init/1 and kthreadd
	- Fix the issues on kthreadd children.
	- Usermode helper created by an unbound workqueue. This shouldn't
	  matter. In the worst case it gives more control to userspace
	  on setting affinity to these short living tasks although this can
	  be tuned with inherited unbound workqueues affinity already.

Fixes: 00b89fe0197f ("sched: Make the idle task quack like a per-CPU kthread")
Reported-by: Paul E. McKenney <paulmck at kernel.org>
Signed-off-by: Frederic Weisbecker <frederic at kernel.org>
Signed-off-by: Ingo Molnar <mingo at kernel.org>
Tested-by: Paul E. McKenney <paulmck at kernel.org>
Link: https://lore.kernel.org/r/20210525235849.441842-1-frederic@kernel.org
(cherry picked from commit a8ea6fc9b089156d9230bfeef964dd9be101a4a9)
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
 kernel/fork.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index da2f7d9a370f..183863e6ea85 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2002,7 +2002,7 @@ static __latent_entropy struct task_struct *copy_process(
 		goto bad_fork_cleanup_count;
 	delayacct_tsk_init(p);	/* Must remain after dup_task_struct() */
-	p->flags &= ~(PF_SUPERPRIV | PF_WQ_WORKER | PF_IDLE);
 	p->flags |= PF_FORKNOEXEC;

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