kthread: Prevent unpark race which puts threads on the wrong cpu

The smpboot threads rely on the park/unpark mechanism which binds per
cpu threads on a particular core. Though the functionality is racy:

CPU0	       	 	CPU1  	     	    CPU2
unpark(T)				    wake_up_process(T)
  clear(SHOULD_PARK)	T runs
			leave parkme() due to !SHOULD_PARK
  bind_to(CPU2)		BUG_ON(wrong CPU)

We cannot let the tasks move themself to the target CPU as one of
those tasks is actually the migration thread itself, which requires
that it starts running on the target cpu right away.

The solution to this problem is to prevent wakeups in park mode which
are not from unpark(). That way we can guarantee that the association
of the task to the target cpu is working correctly.

Add a new task state (TASK_PARKED) which prevents other wakeups and
use this state explicitly for the unpark wakeup.

Peter noticed: Also, since the task state is visible to userspace and
all the parked tasks are still in the PID space, its a good hint in ps
and friends that these tasks aren't really there for the moment.

The migration thread has another related issue.

CPU0	      	     	 CPU1
Bring up CPU2
create_thread(T)
park(T)
 wait_for_completion()
			 parkme()
			 complete()
sched_set_stop_task()
			 schedule(TASK_PARKED)

The sched_set_stop_task() call is issued while the task is on the
runqueue of CPU1 and that confuses the hell out of the stop_task class
on that cpu. So we need the same synchronizaion before
sched_set_stop_task().

Reported-by: Dave Jones <davej@redhat.com>
Reported-and-tested-by: Dave Hansen <dave@sr71.net>
Reported-and-tested-by: Borislav Petkov <bp@alien8.de>
Acked-by: Peter Ziljstra <peterz@infradead.org>
Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Cc: dhillf@gmail.com
Cc: Ingo Molnar <mingo@kernel.org>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/alpine.LFD.2.02.1304091635430.21884@ionos
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/fs/proc/array.c b/fs/proc/array.c
index f7ed9ee..cbd0f1b 100644 (file)
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -143,6 +143,7 @@ static const char * const task_state_array[] = {
        "x (dead)",             /*  64 */
        "K (wakekill)",         /* 128 */
        "W (waking)",           /* 256 */
+       "P (parked)",           /* 512 */
 };
 
 static inline const char *get_task_state(struct task_struct *tsk)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index d35d2b6..e692a02 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -163,9 +163,10 @@ print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
 #define TASK_DEAD              64
 #define TASK_WAKEKILL          128
 #define TASK_WAKING            256
-#define TASK_STATE_MAX         512
+#define TASK_PARKED            512
+#define TASK_STATE_MAX         1024
 
-#define TASK_STATE_TO_CHAR_STR "RSDTtZXxKW"
+#define TASK_STATE_TO_CHAR_STR "RSDTtZXxKWP"
 
 extern char ___assert_task_state[1 - 2*!!(
                sizeof(TASK_STATE_TO_CHAR_STR)-1 != ilog2(TASK_STATE_MAX)+1)];

diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
index 5a8671e..e5586ca 100644 (file)
--- a/include/trace/events/sched.h
+++ b/include/trace/events/sched.h
@@ -147,7 +147,7 @@ TRACE_EVENT(sched_switch,
                  __print_flags(__entry->prev_state & (TASK_STATE_MAX-1), "|",
                                { 1, "S"} , { 2, "D" }, { 4, "T" }, { 8, "t" },
                                { 16, "Z" }, { 32, "X" }, { 64, "x" },
-                               { 128, "W" }) : "R",
+                               { 128, "K" }, { 256, "W" }, { 512, "P" }) : "R",
                __entry->prev_state & TASK_STATE_MAX ? "+" : "",
                __entry->next_comm, __entry->next_pid, __entry->next_prio)
 );

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 691dc2e..9eb7fed 100644 (file)
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -124,12 +124,12 @@ void *kthread_data(struct task_struct *task)
 
 static void __kthread_parkme(struct kthread *self)
 {
-       __set_current_state(TASK_INTERRUPTIBLE);
+       __set_current_state(TASK_PARKED);
        while (test_bit(KTHREAD_SHOULD_PARK, &self->flags)) {
                if (!test_and_set_bit(KTHREAD_IS_PARKED, &self->flags))
                        complete(&self->parked);
                schedule();
-               __set_current_state(TASK_INTERRUPTIBLE);
+               __set_current_state(TASK_PARKED);
        }
        clear_bit(KTHREAD_IS_PARKED, &self->flags);
        __set_current_state(TASK_RUNNING);
@@ -256,8 +256,13 @@ struct task_struct *kthread_create_on_node(int (*threadfn)(void *data),
 }
 EXPORT_SYMBOL(kthread_create_on_node);
 
-static void __kthread_bind(struct task_struct *p, unsigned int cpu)
+static void __kthread_bind(struct task_struct *p, unsigned int cpu, long state)
 {
+       /* Must have done schedule() in kthread() before we set_task_cpu */
+       if (!wait_task_inactive(p, state)) {
+               WARN_ON(1);
+               return;
+       }
        /* It's safe because the task is inactive. */
        do_set_cpus_allowed(p, cpumask_of(cpu));
        p->flags |= PF_THREAD_BOUND;
@@ -274,12 +279,7 @@ static void __kthread_bind(struct task_struct *p, unsigned int cpu)
  */
 void kthread_bind(struct task_struct *p, unsigned int cpu)
 {
-       /* Must have done schedule() in kthread() before we set_task_cpu */
-       if (!wait_task_inactive(p, TASK_UNINTERRUPTIBLE)) {
-               WARN_ON(1);
-               return;
-       }
-       __kthread_bind(p, cpu);
+       __kthread_bind(p, cpu, TASK_UNINTERRUPTIBLE);
 }
 EXPORT_SYMBOL(kthread_bind);
 
@@ -324,6 +324,22 @@ static struct kthread *task_get_live_kthread(struct task_struct *k)
        return NULL;
 }
 
+static void __kthread_unpark(struct task_struct *k, struct kthread *kthread)
+{
+       clear_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
+       /*
+        * We clear the IS_PARKED bit here as we don't wait
+        * until the task has left the park code. So if we'd
+        * park before that happens we'd see the IS_PARKED bit
+        * which might be about to be cleared.
+        */
+       if (test_and_clear_bit(KTHREAD_IS_PARKED, &kthread->flags)) {
+               if (test_bit(KTHREAD_IS_PER_CPU, &kthread->flags))
+                       __kthread_bind(k, kthread->cpu, TASK_PARKED);
+               wake_up_state(k, TASK_PARKED);
+       }
+}
+
 /**
  * kthread_unpark - unpark a thread created by kthread_create().
  * @k:         thread created by kthread_create().
@@ -336,20 +352,8 @@ void kthread_unpark(struct task_struct *k)
 {
        struct kthread *kthread = task_get_live_kthread(k);
 
-       if (kthread) {
-               clear_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
-               /*
-                * We clear the IS_PARKED bit here as we don't wait
-                * until the task has left the park code. So if we'd
-                * park before that happens we'd see the IS_PARKED bit
-                * which might be about to be cleared.
-                */
-               if (test_and_clear_bit(KTHREAD_IS_PARKED, &kthread->flags)) {
-                       if (test_bit(KTHREAD_IS_PER_CPU, &kthread->flags))
-                               __kthread_bind(k, kthread->cpu);
-                       wake_up_process(k);
-               }
-       }
+       if (kthread)
+               __kthread_unpark(k, kthread);
        put_task_struct(k);
 }
 
@@ -407,7 +411,7 @@ int kthread_stop(struct task_struct *k)
        trace_sched_kthread_stop(k);
        if (kthread) {
                set_bit(KTHREAD_SHOULD_STOP, &kthread->flags);
-               clear_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
+               __kthread_unpark(k, kthread);
                wake_up_process(k);
                wait_for_completion(&kthread->exited);
        }

diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index 8eaed9a..02fc5c9 100644 (file)
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -185,8 +185,18 @@ __smpboot_create_thread(struct smp_hotplug_thread *ht, unsigned int cpu)
        }
        get_task_struct(tsk);
        *per_cpu_ptr(ht->store, cpu) = tsk;
-       if (ht->create)
-               ht->create(cpu);
+       if (ht->create) {
+               /*
+                * Make sure that the task has actually scheduled out
+                * into park position, before calling the create
+                * callback. At least the migration thread callback
+                * requires that the task is off the runqueue.
+                */
+               if (!wait_task_inactive(tsk, TASK_PARKED))
+                       WARN_ON(1);
+               else
+                       ht->create(cpu);
+       }
        return 0;
 }