[PATCH 4.17 171/324] kthread, sched/core: Fix kthread_parkme() (again...)

Greg Kroah-Hartman
2018-08-23T08:52:52Z

From: Peter Zijlstra <peterz@infradead.org>

[ Upstream commit 1cef1150ef40ec52f507436a14230cbc2623299c ]

Gaurav reports that commit:

85f1abe0019f ("kthread, sched/wait: Fix kthread_parkme() completion issue")

isn't working for him. Because of the following race:

> controller Thread CPUHP Thread
> takedown_cpu
> kthread_park
> kthread_parkme
> Set KTHREAD_SHOULD_PARK
> smpboot_thread_fn
> set Task interruptible
>
>
> wake_up_process
> if (!(p->state & state))
> goto out;
>
> Kthread_parkme
> SET TASK_PARKED
> schedule
> raw_spin_lock(&rq->lock)
> ttwu_remote
> waiting for __task_rq_lock
> context_switch
>
> finish_lock_switch
>
>
>
> Case TASK_PARKED
> kthread_park_complete
>
>
> SET Running

Furthermore, Oleg noticed that the whole scheduler TASK_PARKED
handling is buggered because the TASK_DEAD thing is done with
preemption disabled, the current code can still complete early on
preemption :/

So basically revert that earlier fix and go with a variant of the
alternative mentioned in the commit. Promote TASK_PARKED to special
state to avoid the store-store issue on task->state leading to the
WARN in kthread_unpark() -> __kthread_bind().

But in addition, add wait_task_inactive() to kthread_park() to ensure
the task really is PARKED when we return from kthread_park(). This
avoids the whole kthread still gets migrated nonsense -- although it
would be really good to get this done differently.

Reported-by: Gaurav Kohli <gkohli@codeaurora.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 85f1abe0019f ("kthread, sched/wait: Fix kthread_parkme() completion issue")
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/linux/kthread.h | 1 -
include/linux/sched.h | 2 +-
kernel/kthread.c | 30 ++++++++++++++++++++++++------
kernel/sched/core.c | 31 +++++++++++--------------------
4 files changed, 36 insertions(+), 28 deletions(-)

--- a/include/linux/kthread.h
+++ b/include/linux/kthread.h
@@ -62,7 +62,6 @@ void *kthread_probe_data(struct task_str
int kthread_park(struct task_struct *k);
void kthread_unpark(struct task_struct *k);
void kthread_parkme(void);
-void kthread_park_complete(struct task_struct *k);

int kthreadd(void *unused);
extern struct task_struct *kthreadd_task;
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -117,7 +117,7 @@ struct task_group;
* the comment with set_special_state().
*/
#define is_special_task_state(state) \
- ((state) & (__TASK_STOPPED | __TASK_TRACED | TASK_DEAD))
+ ((state) & (__TASK_STOPPED | __TASK_TRACED | TASK_PARKED | TASK_DEAD))

#define __set_current_state(state_value) \
do { \
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -177,9 +177,20 @@ void *kthread_probe_data(struct task_str
static void __kthread_parkme(struct kthread *self)
{
for (;;) {
- set_current_state(TASK_PARKED);
+ /*
+ * TASK_PARKED is a special state; we must serialize against
+ * possible pending wakeups to avoid store-store collisions on
+ * task->state.
+ *
+ * Such a collision might possibly result in the task state
+ * changin from TASK_PARKED and us failing the
+ * wait_task_inactive() in kthread_park().
+ */
+ set_special_state(TASK_PARKED);
if (!test_bit(KTHREAD_SHOULD_PARK, &self->flags))
break;
+
+ complete_all(&self->parked);
schedule();
}
__set_current_state(TASK_RUNNING);
@@ -191,11 +202,6 @@ void kthread_parkme(void)
}
EXPORT_SYMBOL_GPL(kthread_parkme);

-void kthread_park_complete(struct task_struct *k)
-{
- complete_all(&to_kthread(k)->parked);
-}
-
static int kthread(void *_create)
{
/* Copy data: it's on kthread's stack */
@@ -467,6 +473,9 @@ void kthread_unpark(struct task_struct *

reinit_completion(&kthread->parked);
clear_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
+ /*
+ * __kthread_parkme() will either see !SHOULD_PARK or get the wakeup.
+ */
wake_up_state(k, TASK_PARKED);
}
EXPORT_SYMBOL_GPL(kthread_unpark);
@@ -493,7 +502,16 @@ int kthread_park(struct task_struct *k)
set_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
if (k != current) {
wake_up_process(k);
+ /*
+ * Wait for __kthread_parkme() to complete(), this means we
+ * _will_ have TASK_PARKED and are about to call schedule().
+ */
wait_for_completion(&kthread->parked);
+ /*
+ * Now wait for that schedule() to complete and the task to
+ * get scheduled out.
+ */
+ WARN_ON_ONCE(!wait_task_inactive(k, TASK_PARKED));
}

return 0;
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7,7 +7,6 @@
*/
#include "sched.h"

-#include <linux/kthread.h>
#include <linux/nospec.h>

#include <asm/switch_to.h>
@@ -2738,28 +2737,20 @@ static struct rq *finish_task_switch(str
membarrier_mm_sync_core_before_usermode(mm);
mmdrop(mm);
}
- if (unlikely(prev_state & (TASK_DEAD|TASK_PARKED))) {
- switch (prev_state) {
- case TASK_DEAD:
- if (prev->sched_class->task_dead)
- prev->sched_class->task_dead(prev);
-
- /*
- * Remove function-return probe instances associated with this
- * task and put them back on the free list.
- */
- kprobe_flush_task(prev);
+ if (unlikely(prev_state == TASK_DEAD)) {
+ if (prev->sched_class->task_dead)
+ prev->sched_class->task_dead(prev);

- /* Task is done with its stack. */
- put_task_stack(prev);
+ /*
+ * Remove function-return probe instances associated with this
+ * task and put them back on the free list.
+ */
+ kprobe_flush_task(prev);

- put_task_struct(prev);
- break;
+ /* Task is done with its stack. */
+ put_task_stack(prev);

- case TASK_PARKED:
- kthread_park_complete(prev);
- break;
- }
+ put_task_struct(prev);
}

tick_nohz_task_switch();