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Commit b5bf9a90 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

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: default avatarGaurav Kohli <gkohli@codeaurora.org>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarOleg 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: default avatarIngo Molnar <mingo@kernel.org>
parent 85f1abe0

include/linux/sched.h

+45 −5
Original line number Diff line number Diff line
@@ -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
@@ -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: */

include/linux/sched/signal.h

+1 −1
Original line number Diff line number Diff line
@@ -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();

kernel/sched/core.c

+1 −16
Original line number Diff line number Diff line
@@ -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;

kernel/signal.c

+15 −2
Original line number Diff line number Diff line
@@ -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);



		/*