Re: [PATCH] kthread/smpboot: Serialize kthread parking against wakeup

Peter Zijlstra
2018-04-24T18:05:21Z

On Tue, Apr 24, 2018 at 08:12:49PM +0530, Kohli, Gaurav wrote:
> @@ -157,6 +156,7 @@ static int smpboot_thread_fn(void *data)
>
>                 if (!ht->thread_should_run(td->cpu)) {
>
>                         preempt_enable_no_resched();
>
> +                       set_current_state(TASK_INTERRUPTIBLE);
>
>                         schedule();
>
>                 } else {
>
>                         __set_current_state(TASK_RUNNING);
>
> Please suggest if this approach is better.

Bah, my brain isn't working... see below for the 'correct' version of
your second patch.

But this violates the normal pattern; see the comment near
set_current_state(). That pattern ensures the thread either sees the
wakeup condition or the actual wakeup.

I'm thinking that with this patch there is a scenario where we'll miss
both the kthread_should_park() and the actual wakeup and end up not
doing anything.

I do the like the end result, but I suspect it's buggy.

---
kernel/smpboot.c | 7 +------
1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index 5043e7433f4b..5bdf57f2ce68 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -109,10 +109,8 @@ static int smpboot_thread_fn(void *data)
struct smp_hotplug_thread *ht = td->ht;

while (1) {
- set_current_state(TASK_INTERRUPTIBLE);
preempt_disable();
if (kthread_should_stop()) {
- __set_current_state(TASK_RUNNING);
preempt_enable();
/* cleanup must mirror setup */
if (ht->cleanup && td->status != HP_THREAD_NONE)
@@ -122,7 +120,6 @@ static int smpboot_thread_fn(void *data)
}

if (kthread_should_park()) {
- __set_current_state(TASK_RUNNING);
preempt_enable();
if (ht->park && td->status == HP_THREAD_ACTIVE) {
BUG_ON(td->cpu != smp_processor_id());
@@ -139,7 +136,6 @@ static int smpboot_thread_fn(void *data)
/* Check for state change setup */
switch (td->status) {
case HP_THREAD_NONE:
- __set_current_state(TASK_RUNNING);
preempt_enable();
if (ht->setup)
ht->setup(td->cpu);
@@ -147,7 +143,6 @@ static int smpboot_thread_fn(void *data)
continue;

case HP_THREAD_PARKED:
- __set_current_state(TASK_RUNNING);
preempt_enable();
if (ht->unpark)
ht->unpark(td->cpu);
@@ -156,10 +151,10 @@ static int smpboot_thread_fn(void *data)
}

if (!ht->thread_should_run(td->cpu)) {
+ set_current_state(TASK_IDLE);
preempt_enable_no_resched();
schedule();
} else {
- __set_current_state(TASK_RUNNING);
preempt_enable();
ht->thread_fn(td->cpu);
}