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Commit fbad3218 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Greg Kroah-Hartman
Browse files

kthread: Extract KTHREAD_IS_PER_CPU 



[ Upstream commit ac687e6e8c26181a33270efd1a2e2241377924b0 ]

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


Signed-off-by: default avatarSasha Levin <sashal@kernel.org>
parent 52b4c58b

include/linux/kthread.h

+3 −0
Original line number Diff line number Diff line
@@ -32,6 +32,9 @@ 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).

kernel/kthread.c

+26 −1
Original line number Diff line number Diff line
@@ -460,11 +460,36 @@ 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().

kernel/smpboot.c

+1 −0
Original line number Diff line number Diff line
@@ -187,6 +187,7 @@ __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.