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Message-ID: <20161031200823.GC19430@redhat.com>
Date: Mon, 31 Oct 2016 21:08:23 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Andy Lutomirski <luto@...capital.net>,
Roman Pen <roman.penyaev@...fitbricks.com>,
Andy Lutomirski <luto@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Chunming Zhou <David1.Zhou@....com>,
Alex Deucher <alexander.deucher@....com>
Subject: [PATCH 2/2] kthread: don't use to_live_kthread() in kthread_park()
and kthread_unpark()
Now that to_kthread() is always valid we can change kthread_park() and
kthread_unpark() to use it and kill to_live_kthread().
kthread_unpark() is trivial, if KTHREAD_IS_PARKED is set we know that this
kthread has called complete(&self->parked), we do not care if it exits after
that if we race with kthread_stop().
kthread_park() is more tricky, but only because its semantics is not well
defined. It returns -ENOSYS if the thread exited but this can never happen
and as Roman pointed out kthread_park() can obviously block forever if it
could race with the exiting kthread.
I think we need to unexport kthread_park/unpark, and either make it return
"void" or actually fix the race with kthred_stop/exit. This patch just adds
WARN_ON(PF_EXITING) for now.
The usage of kthread_park() in cpuhp code (cpu.c, smpboot.c, stop_machine.c)
is fine. It can never see an exiting/exited kthread, smpboot_destroy_threads()
clears *ht->store, smpboot_park_thread() checks it is not NULL under the same
smpboot_threads_lock. cpuhp_threads and cpu_stop_threads never exit, so other
callers are fine too.
But it has two more users:
- watchdog_park_threads() and it does not look nice. The code is actually
correct, get_online_cpus() ensures that kthread_park() can't race with
itself (note that kthread_park() can't handle this race correctly), but
imo it should not use kthread_park() directly.
- drivers/gpu/drm/amd/scheduler/gpu_scheduler.c and I think it should not
use kthread_park() too.
But this patch should not break this code. kthread_park() must not be
called after amd_sched_fini() which does kthread_stop(), otherwise even
to_live_kthread() is not safe because task_struct can be already freed
and sched->thread can point to nowhere.
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
kernel/kthread.c | 69 ++++++++++++++++++++------------------------------------
1 file changed, 24 insertions(+), 45 deletions(-)
diff --git a/kernel/kthread.c b/kernel/kthread.c
index 4dcbc8b..01d2716 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -78,21 +78,6 @@ void free_kthread_struct(struct task_struct *k)
kfree(to_kthread(k));
}
-#define __to_kthread(vfork) \
- container_of(vfork, struct kthread, exited)
-
-/*
- * TODO: kill it and use to_kthread(). But we still need the users
- * like kthread_stop() which has to sync with the exiting kthread.
- */
-static struct kthread *to_live_kthread(struct task_struct *k)
-{
- struct completion *vfork = ACCESS_ONCE(k->vfork_done);
- if (likely(vfork))
- return __to_kthread(vfork);
- return NULL;
-}
-
/**
* kthread_should_stop - should this kthread return now?
*
@@ -441,8 +426,18 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
return p;
}
-static void __kthread_unpark(struct task_struct *k, struct kthread *kthread)
+/**
+ * kthread_unpark - unpark a thread created by kthread_create().
+ * @k: thread created by kthread_create().
+ *
+ * Sets kthread_should_park() for @k to return false, wakes it, and
+ * waits for it to return. If the thread is marked percpu then its
+ * bound to the cpu again.
+ */
+void kthread_unpark(struct task_struct *k)
{
+ struct kthread *kthread = to_kthread(k);
+
clear_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
/*
* We clear the IS_PARKED bit here as we don't wait
@@ -460,22 +455,6 @@ static void __kthread_unpark(struct task_struct *k, struct kthread *kthread)
wake_up_state(k, TASK_PARKED);
}
}
-
-/**
- * kthread_unpark - unpark a thread created by kthread_create().
- * @k: thread created by kthread_create().
- *
- * Sets kthread_should_park() for @k to return false, wakes it, and
- * waits for it to return. If the thread is marked percpu then its
- * bound to the cpu again.
- */
-void kthread_unpark(struct task_struct *k)
-{
- struct kthread *kthread = to_live_kthread(k);
-
- if (kthread)
- __kthread_unpark(k, kthread);
-}
EXPORT_SYMBOL_GPL(kthread_unpark);
/**
@@ -492,20 +471,20 @@ EXPORT_SYMBOL_GPL(kthread_unpark);
*/
int kthread_park(struct task_struct *k)
{
- struct kthread *kthread = to_live_kthread(k);
- int ret = -ENOSYS;
-
- if (kthread) {
- if (!test_bit(KTHREAD_IS_PARKED, &kthread->flags)) {
- set_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
- if (k != current) {
- wake_up_process(k);
- wait_for_completion(&kthread->parked);
- }
+ struct kthread *kthread = to_kthread(k);
+
+ if (WARN_ON(k->flags & PF_EXITING))
+ return -ENOSYS;
+
+ if (!test_bit(KTHREAD_IS_PARKED, &kthread->flags)) {
+ set_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
+ if (k != current) {
+ wake_up_process(k);
+ wait_for_completion(&kthread->parked);
}
- ret = 0;
}
- return ret;
+
+ return 0;
}
EXPORT_SYMBOL_GPL(kthread_park);
@@ -534,7 +513,7 @@ int kthread_stop(struct task_struct *k)
get_task_struct(k);
kthread = to_kthread(k);
set_bit(KTHREAD_SHOULD_STOP, &kthread->flags);
- __kthread_unpark(k, kthread);
+ kthread_unpark(k);
wake_up_process(k);
wait_for_completion(&kthread->exited);
ret = k->exit_code;
--
2.5.0
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