[PATCH 4.14 117/189] sched/core: Introduce set_special_state()

Greg Kroah-Hartman
2018-06-18T08:55:08Z

From: Peter Zijlstra <peterz@infradead.org>

[ Upstream commit b5bf9a90bbebffba888c9144c5a8a10317b04064 ]

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>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/linux/sched.h | 50 ++++++++++++++++++++++++++++++++++++++-----
include/linux/sched/signal.h | 2 -
kernel/sched/core.c | 17 --------------
kernel/signal.c | 17 ++++++++++++--
4 files changed, 62 insertions(+), 24 deletions(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -113,17 +113,36 @@ 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
@@ -145,8 +164,8 @@ 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.
@@ -155,12 +174,33 @@ 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: */
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -280,7 +280,7 @@ static inline void kernel_signal_stop(vo
{
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();
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3374,23 +3374,8 @@ static void __sched notrace __schedule(b

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;
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1828,14 +1828,27 @@ static void ptrace_stop(int exit_code, i
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;
@@ -2043,7 +2056,7 @@ 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);

/*