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Commit 150593bf authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Ingo Molnar
Browse files

sched/api: Introduce task_rcu_dereference() and try_get_task_struct() 



Generally task_struct is only protected by RCU if it was found on a
RCU protected list (say, for_each_process() or find_task_by_vpid()).

As Kirill pointed out rq->curr isn't protected by RCU, the scheduler
drops the (potentially) last reference without RCU gp, this means
that we need to fix the code which uses foreign_rq->curr under
rcu_read_lock().

Add a new helper which can be used to dereference rq->curr or any
other pointer to task_struct assuming that it should be cleared or
updated before the final put_task_struct(). It returns non-NULL
only if this task can't go away before rcu_read_unlock().

( Also add try_get_task_struct() to make it easier to use this API
  correctly. )

Suggested-by: default avatarKirill Tkhai <ktkhai@parallels.com>
Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
[ Updated comments; added try_get_task_struct()]
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Kirill Tkhai <tkhai@yandex.ru>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Link: http://lkml.kernel.org/r/20160518170218.GY3192@twins.programming.kicks-ass.net


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent df55f462

include/linux/sched.h

+3 −0
Original line number Original line Diff line number Diff line
@@ -2139,6 +2139,9 @@ static inline void put_task_struct(struct task_struct *t) 
		__put_task_struct(t);

		__put_task_struct(t);

}

}





struct task_struct *task_rcu_dereference(struct task_struct **ptask);

struct task_struct *try_get_task_struct(struct task_struct **ptask);



#ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN

#ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN

extern void task_cputime(struct task_struct *t,

extern void task_cputime(struct task_struct *t,

			 cputime_t *utime, cputime_t *stime);

			 cputime_t *utime, cputime_t *stime);

kernel/exit.c

+76 −0
Original line number Original line Diff line number Diff line
@@ -210,6 +210,82 @@ void release_task(struct task_struct *p) 
		goto repeat;

		goto repeat;

}

}





/*

 * Note that if this function returns a valid task_struct pointer (!NULL)

 * task->usage must remain >0 for the duration of the RCU critical section.

 */

struct task_struct *task_rcu_dereference(struct task_struct **ptask)

{

	struct sighand_struct *sighand;

	struct task_struct *task;



	/*

	 * We need to verify that release_task() was not called and thus

	 * delayed_put_task_struct() can't run and drop the last reference

	 * before rcu_read_unlock(). We check task->sighand != NULL,

	 * but we can read the already freed and reused memory.

	 */

retry:

	task = rcu_dereference(*ptask);

	if (!task)

		return NULL;



	probe_kernel_address(&task->sighand, sighand);



	/*

	 * Pairs with atomic_dec_and_test() in put_task_struct(). If this task

	 * was already freed we can not miss the preceding update of this

	 * pointer.

	 */

	smp_rmb();

	if (unlikely(task != READ_ONCE(*ptask)))

		goto retry;



	/*

	 * We've re-checked that "task == *ptask", now we have two different

	 * cases:

	 *

	 * 1. This is actually the same task/task_struct. In this case

	 *    sighand != NULL tells us it is still alive.

	 *

	 * 2. This is another task which got the same memory for task_struct.

	 *    We can't know this of course, and we can not trust

	 *    sighand != NULL.

	 *

	 *    In this case we actually return a random value, but this is

	 *    correct.

	 *

	 *    If we return NULL - we can pretend that we actually noticed that

	 *    *ptask was updated when the previous task has exited. Or pretend

	 *    that probe_slab_address(&sighand) reads NULL.

	 *

	 *    If we return the new task (because sighand is not NULL for any

	 *    reason) - this is fine too. This (new) task can't go away before

	 *    another gp pass.

	 *

	 *    And note: We could even eliminate the false positive if re-read

	 *    task->sighand once again to avoid the falsely NULL. But this case

	 *    is very unlikely so we don't care.

	 */

	if (!sighand)

		return NULL;



	return task;

}



struct task_struct *try_get_task_struct(struct task_struct **ptask)

{

	struct task_struct *task;



	rcu_read_lock();

	task = task_rcu_dereference(ptask);

	if (task)

		get_task_struct(task);

	rcu_read_unlock();



	return task;

}



/*

/*

 * Determine if a process group is "orphaned", according to the POSIX

 * Determine if a process group is "orphaned", according to the POSIX

 * definition in 2.2.2.52.  Orphaned process groups are not to be affected

 * definition in 2.2.2.52.  Orphaned process groups are not to be affected