sched/core: Introduce set_special_state()

Gaurav reported a perceived problem with TASK_PARKED, which turned out
to be a broken wait-loop pattern in __kthread_parkme(), but the
reported issue can (and does) in fact happen for states that do not do
condition based sleeps.

When the 'current->state = TASK_RUNNING' store of a previous
(concurrent) try_to_wake_up() collides with the setting of a 'special'
sleep state, we can loose the sleep state.

Normal condition based wait-loops are immune to this problem, but for
sleep states that are not condition based are subject to this problem.

There already is a fix for TASK_DEAD. Abstract that and also apply it
to TASK_STOPPED and TASK_TRACED, both of which are also without
condition based wait-loop.

Reported-by: Gaurav Kohli <gkohli@codeaurora.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

4 files changed, 62 insertions, 24 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index b3d697f3b573..c2413703f45d 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -112,17 +112,36 @@ struct task_group;
 
 #ifdef CONFIG_DEBUG_ATOMIC_SLEEP
 
+/*
+ * Special states are those that do not use the normal wait-loop pattern. See
+ * the comment with set_special_state().
+ */
+#define is_special_task_state(state)                            \
+        ((state) & (__TASK_STOPPED | __TASK_TRACED | TASK_DEAD))
+
 #define __set_current_state(state_value)                        \
         do {                                                    \
+                WARN_ON_ONCE(is_special_task_state(state_value));\
                 current->task_state_change = _THIS_IP_;         \
                 current->state = (state_value);                 \
         } while (0)
+
 #define set_current_state(state_value)                          \
         do {                                                    \
+                WARN_ON_ONCE(is_special_task_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_task_state(state_value));      \
+                raw_spin_lock_irqsave(&current->pi_lock, flags);        \
+                current->task_state_change = _THIS_IP_;                 \
+                current->state = (state_value);                         \
+                raw_spin_unlock_irqrestore(&current->pi_lock, flags);   \
+        } while (0)
 #else
 /*
  * set_current_state() includes a barrier so that the write of current->state
@@ -144,8 +163,8 @@ struct task_group;
  *
  * The above is typically ordered against the wakeup, which does:
  *
- *      need_sleep = false;
- *      wake_up_state(p, TASK_UNINTERRUPTIBLE);
+ *   need_sleep = false;
+ *   wake_up_state(p, TASK_UNINTERRUPTIBLE);
  *
  * Where wake_up_state() (and all other wakeup primitives) imply enough
  * barriers to order the store of the variable against wakeup.
@@ -154,12 +173,33 @@ 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))
+#define __set_current_state(state_value)                                \
+        current->state = (state_value)
+
+#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 our state change.
+ */
+#define set_special_state(state_value)                                  \
+        do {                                                            \
+                unsigned long flags; /* may shadow */                   \
+                raw_spin_lock_irqsave(&current->pi_lock, flags);        \
+                current->state = (state_value);                         \
+                raw_spin_unlock_irqrestore(&current->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(&current->sighand->siglock);
         if (current->jobctl & JOBCTL_STOP_DEQUEUED)
-                __set_current_state(TASK_STOPPED);
+                set_special_state(TASK_STOPPED);
         spin_unlock_irq(&current->sighand->siglock);
 
         schedule();
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7ad60e00a6a8..ffde9eebc846 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3508,23 +3508,8 @@ 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(&current->pi_lock);
-        raw_spin_unlock_irq(&current->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..9c33163a6165 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1961,14 +1961,27 @@ static void ptrace_stop(int exit_code, int why, int clear_code, siginfo_t *info)
                         return;
         }
 
+        set_special_state(TASK_TRACED);
+
         /*
          * We're committing to trapping.  TRACED should be visible before
          * TRAPPING is cleared; otherwise, the tracer might fail do_wait().
          * Also, transition to TRACED and updates to ->jobctl should be
          * atomic with respect to siglock and should be done after the arch
          * hook as siglock is released and regrabbed across it.
+         *
+         *     TRACER                               TRACEE
+         *
+         *     ptrace_attach()
+         * [L]   wait_on_bit(JOBCTL_TRAPPING)   [S] set_special_state(TRACED)
+         *     do_wait()
+         *       set_current_state()                smp_wmb();
+         *       ptrace_do_wait()
+         *         wait_task_stopped()
+         *           task_stopped_code()
+         * [L]         task_is_traced()         [S] task_clear_jobctl_trapping();
          */
-        set_current_state(TASK_TRACED);
+        smp_wmb();
 
         current->last_siginfo = info;
         current->exit_code = exit_code;
@@ -2176,7 +2189,7 @@ 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(&current->sighand->siglock);
 
                 /*