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Date: Thu, 26 Apr 2018 10:41:31 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Gaurav Kohli <gkohli@...eaurora.org>
Cc: tglx@...utronix.de, mpe@...erman.id.au, mingo@...nel.org,
bigeasy@...utronix.de, linux-kernel@...r.kernel.org,
linux-arm-msm@...r.kernel.org,
Neeraj Upadhyay <neeraju@...eaurora.org>,
Will Deacon <will.deacon@....com>,
Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against
wakeup
On Wed, Apr 25, 2018 at 10:09:17PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 25, 2018 at 02:03:19PM +0530, Gaurav Kohli wrote:
> > diff --git a/kernel/smpboot.c b/kernel/smpboot.c
> > index 5043e74..c5c5184 100644
> > --- a/kernel/smpboot.c
> > +++ b/kernel/smpboot.c
> > @@ -122,7 +122,45 @@ static int smpboot_thread_fn(void *data)
> > }
> >
> > if (kthread_should_park()) {
> > + /*
> > + * Serialize against wakeup.
> *
> * Prior wakeups must complete and later wakeups
> * will observe TASK_RUNNING.
> *
> * This avoids the case where the TASK_RUNNING
> * store from ttwu() competes with the
> * TASK_PARKED store from kthread_parkme().
> *
> * If the TASK_PARKED store looses that
> * competition, kthread_unpark() will go wobbly.
> > + */
> > + raw_spin_lock(¤t->pi_lock);
> > __set_current_state(TASK_RUNNING);
> > + raw_spin_unlock(¤t->pi_lock);
> > preempt_enable();
> > if (ht->park && td->status == HP_THREAD_ACTIVE) {
> > BUG_ON(td->cpu != smp_processor_id());
>
> Does that work for you?
>
> But looking at this a bit more; don't we have the exact same problem
> with the TASK_RUNNING store in the !ht->thread_should_run() case?
> Suppose a ttwu() happens concurrently there, it can end up competing
> against the TASK_INTERRUPTIBLE store, no?
>
> Of course, that race is not fatal, we'll just end up going around the
> loop once again I suppose. Maybe a comment there too?
>
> /*
> * A similar race is possible here, but loosing
> * the TASK_INTERRUPTIBLE store is harmless and
> * will make us go around the loop once more.
> */
>
And with slightly more sleep I realize this is actually the normal
and expected pattern. The comment with __set_current_state() even
mentions this.
Also, I think the above patch is 'wrong'. It is not the TASK_RUNNING
store that is a problem it is the TASK_PARKED state that is special. And
if you look at do_task_dead() you'll see we do something very similar
for setting TASK_DEAD.
It is a problem specific to blocked states that do not follow the normal
wait pattern:
for (;;) {
set_current_state(STATE);
if (cond)
break;
schedule();
}
__set_current_state(RUNNING);
The initial store or STATE can _always_ loose against a competing
RUNNING store from a previous wakeup, but the wait-loop and @cond test
will make it harmless.
The special states (DEAD,STOPPED,..) are different though, they
do not have a loop and expect to be honoured.
This had me looking at __kthread_park() and afaict we actually have a
condition, namely KTHREAD_SHOULD_PARK, which would suggest the following
change:
diff --git a/kernel/kthread.c b/kernel/kthread.c
index cd50e99202b0..4b6503c6a029 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -177,12 +177,13 @@ void *kthread_probe_data(struct task_struct *task)
static void __kthread_parkme(struct kthread *self)
{
- __set_current_state(TASK_PARKED);
- while (test_bit(KTHREAD_SHOULD_PARK, &self->flags)) {
+ for (;;) {
+ __set_task_state(TASK_PARKED);
+ if (!test_bit(KTHREAD_SHOULD_PARK, &self->flags))
+ break;
if (!test_and_set_bit(KTHREAD_IS_PARKED, &self->flags))
complete(&self->parked);
schedule();
- __set_current_state(TASK_PARKED);
}
clear_bit(KTHREAD_IS_PARKED, &self->flags);
__set_current_state(TASK_RUNNING);
For the others, I think we want to do something like the below. I still
need to look at TASK_TRACED, which I suspect is also special, but ptrace
always hurts my brain.
Opinions?
diff --git a/include/linux/sched.h b/include/linux/sched.h
index b3d697f3b573..f4098435a882 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -110,19 +110,45 @@ struct task_group;
(task->flags & PF_FROZEN) == 0 && \
(task->state & TASK_NOLOAD) == 0)
+/*
+ * Special states are those that do not use the normal wait-loop pattern. See
+ * the comment with set_special_state().
+ */
+#define is_special_state(state) \
+ ((state) == TASK_DEAD || \
+ (state) == TASK_STOPPED)
+
#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
+/*
+ * Assert we don't use the regular *set_current_state() helpers for special
+ * states. See the comment with set_special_state().
+ */
+#define assert_special_state(state) WARN_ON_ONCE(is_special_state(state))
+
#define __set_current_state(state_value) \
do { \
+ assert_special_state(state_value); \
current->task_state_change = _THIS_IP_; \
current->state = (state_value); \
} while (0)
+
#define set_current_state(state_value) \
do { \
+ assert_special_state(state_value); \
current->task_state_change = _THIS_IP_; \
smp_store_mb(current->state, (state_value)); \
} while (0)
+#define set_special_state(state_value) \
+ do { \
+ unsigned long flags; /* may shadow */ \
+ WARN_ON_ONCE(!is_special_state(state_value)); \
+ raw_spin_lock_irqsave(¤t->pi_lock, flags); \
+ current->task_state_change = _THIS_IP_; \
+ current->state = (state_value); \
+ raw_spin_unlock_irqrestore(¤t->pi_lock, flags); \
+ } while (0)
#else
/*
* set_current_state() includes a barrier so that the write of current->state
@@ -154,12 +180,30 @@ struct task_group;
* once it observes the TASK_UNINTERRUPTIBLE store the waking CPU can issue a
* TASK_RUNNING store which can collide with __set_current_state(TASK_RUNNING).
*
- * This is obviously fine, since they both store the exact same value.
+ * However, with slightly different timing the wakeup TASK_RUNNING store can
+ * also collide with the TASK_UNINTERRUPTIBLE store. Loosing that store is not
+ * a problem either because that will result in one extra go around the loop
+ * and our @cond test will save the day.
*
* Also see the comments of try_to_wake_up().
*/
#define __set_current_state(state_value) do { current->state = (state_value); } while (0)
#define set_current_state(state_value) smp_store_mb(current->state, (state_value))
+
+/*
+ * set_special_state() should be used for those states when the blocking task
+ * can not use the regular condition based wait-loop. In that case we must
+ * serialize against wakeups such that any possible in-flight TASK_RUNNING stores
+ * will not collide with out state change.
+ */
+#define set_special_state(state_value) \
+ do { \
+ unsigned long flags; /* may shadow */ \
+ raw_spin_lock_irqsave(¤t->pi_lock, flags); \
+ current->state = (state_value); \
+ raw_spin_unlock_irqrestore(¤t->pi_lock, flags); \
+ } while (0)
+
#endif
/* Task command name length: */
diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index a7ce74c74e49..113d1ad1ced7 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -280,7 +280,7 @@ static inline void kernel_signal_stop(void)
{
spin_lock_irq(¤t->sighand->siglock);
if (current->jobctl & JOBCTL_STOP_DEQUEUED)
- __set_current_state(TASK_STOPPED);
+ set_special_state(TASK_STOPPED);
spin_unlock_irq(¤t->sighand->siglock);
schedule();
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5e10aaeebfcc..3898a8047c11 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3498,23 +3498,8 @@ static void __sched notrace __schedule(bool preempt)
void __noreturn do_task_dead(void)
{
- /*
- * The setting of TASK_RUNNING by try_to_wake_up() may be delayed
- * when the following two conditions become true.
- * - There is race condition of mmap_sem (It is acquired by
- * exit_mm()), and
- * - SMI occurs before setting TASK_RUNINNG.
- * (or hypervisor of virtual machine switches to other guest)
- * As a result, we may become TASK_RUNNING after becoming TASK_DEAD
- *
- * To avoid it, we have to wait for releasing tsk->pi_lock which
- * is held by try_to_wake_up()
- */
- raw_spin_lock_irq(¤t->pi_lock);
- raw_spin_unlock_irq(¤t->pi_lock);
-
/* Causes final put_task_struct in finish_task_switch(): */
- __set_current_state(TASK_DEAD);
+ set_special_state(TASK_DEAD);
/* Tell freezer to ignore us: */
current->flags |= PF_NOFREEZE;
diff --git a/kernel/signal.c b/kernel/signal.c
index d4ccea599692..c9cac52b1369 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2176,7 +2176,7 @@ static bool do_signal_stop(int signr)
if (task_participate_group_stop(current))
notify = CLD_STOPPED;
- __set_current_state(TASK_STOPPED);
+ set_special_state(TASK_STOPPED);
spin_unlock_irq(¤t->sighand->siglock);
/*
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