From: Peter Zijlstra <peterz@infradead.org>
Date: Tue, 12 Jan 2021 11:24:04 +0100
Subject: kthread: Extract KTHREAD_IS_PER_CPU
Git-commit: ac687e6e8c26181a33270efd1a2e2241377924b0
Patch-mainline: v5.11-rc5
References: bsc#1204753

There is a need to distinguish geniune per-cpu kthreads from kthreads
that happen to have a single CPU affinity.

Geniune per-cpu kthreads are kthreads that are CPU affine for
correctness, these will obviously have PF_KTHREAD set, but must also
have PF_NO_SETAFFINITY set, lest userspace modify their affinity and
ruins things.

However, these two things are not sufficient, PF_NO_SETAFFINITY is
also set on other tasks that have their affinities controlled through
other means, like for instance workqueues.

Therefore another bit is needed; it turns out kthread_create_per_cpu()
already has such a bit: KTHREAD_IS_PER_CPU, which is used to make
kthread_park()/kthread_unpark() work correctly.

Expose this flag and remove the implicit setting of it from
kthread_create_on_cpu(); the io_uring usage of it seems dubious at
best.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20210121103506.557620262@infradead.org

Acked-by: Michal Koutný <mkoutny@suse.com>
---
 include/linux/kthread.h |  3 +++
 kernel/kthread.c        | 27 ++++++++++++++++++++++++++-
 kernel/smpboot.c        |  1 +
 3 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/include/linux/kthread.h b/include/linux/kthread.h
index 65b81e0c494d..2484ed97e72f 100644
--- a/include/linux/kthread.h
+++ b/include/linux/kthread.h
@@ -33,6 +33,9 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
 					  unsigned int cpu,
 					  const char *namefmt);
 
+void kthread_set_per_cpu(struct task_struct *k, int cpu);
+bool kthread_is_per_cpu(struct task_struct *k);
+
 /**
  * kthread_run - create and wake a thread.
  * @threadfn: the function to run until signal_pending(current).
diff --git a/kernel/kthread.c b/kernel/kthread.c
index a5eceecd4513..e0e4a423f184 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -493,11 +493,36 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
 		return p;
 	kthread_bind(p, cpu);
 	/* CPU hotplug need to bind once again when unparking the thread. */
-	set_bit(KTHREAD_IS_PER_CPU, &to_kthread(p)->flags);
 	to_kthread(p)->cpu = cpu;
 	return p;
 }
 
+void kthread_set_per_cpu(struct task_struct *k, int cpu)
+{
+	struct kthread *kthread = to_kthread(k);
+	if (!kthread)
+		return;
+
+	WARN_ON_ONCE(!(k->flags & PF_NO_SETAFFINITY));
+
+	if (cpu < 0) {
+		clear_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
+		return;
+	}
+
+	kthread->cpu = cpu;
+	set_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
+}
+
+bool kthread_is_per_cpu(struct task_struct *k)
+{
+	struct kthread *kthread = to_kthread(k);
+	if (!kthread)
+		return false;
+
+	return test_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
+}
+
 /**
  * kthread_unpark - unpark a thread created by kthread_create().
  * @k:		thread created by kthread_create().
diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index 2efe1e206167..f25208e8df83 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -188,6 +188,7 @@ __smpboot_create_thread(struct smp_hotplug_thread *ht, unsigned int cpu)
 		kfree(td);
 		return PTR_ERR(tsk);
 	}
+	kthread_set_per_cpu(tsk, cpu);
 	/*
 	 * Park the thread so that it could start right on the CPU
 	 * when it is available.