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Commit d2e3ee9b authored by Oleg Nesterov's avatar Oleg Nesterov Committed by James Morris
Browse files

kernel: fix is_single_threaded 



- Fix the comment, is_single_threaded(p) actually means that nobody shares
  ->mm with p.

  I think this helper should be renamed, and it should not have arguments.
  With or without this patch it must not be used unless p == current,
  otherwise we can't safely use p->signal or p->mm.

- "if (atomic_read(&p->signal->count) != 1)" is not right when we have a
  zombie group leader, use signal->live instead.

- Add PF_KTHREAD check to skip kernel threads which may borrow p->mm,
  otherwise we can return the wrong "false".

- Use for_each_process() instead of do_each_thread(), all threads must use
  the same ->mm.

- Use down_write(mm->mmap_sem) + rcu_read_lock() instead of tasklist_lock
  to iterate over the process list. If there is another CLONE_VM process
  it can't pass exit_mm() which takes the same mm->mmap_sem. We can miss
  a freshly forked CLONE_VM task, but this doesn't matter because we must
  see its parent and return false.

Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: James Morris <jmorris@namei.org>
Cc: Roland McGrath <roland@redhat.com>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarJames Morris <jmorris@namei.org>
parent 713c0ecd

lib/is_single_threaded.c

+36 −26
Original line number Diff line number Diff line
@@ -12,34 +12,44 @@ 


#include <linux/sched.h>



/**

 * is_single_threaded - Determine if a thread group is single-threaded or not

 * @p: A task in the thread group in question

 *

 * This returns true if the thread group to which a task belongs is single

 * threaded, false if it is not.

/*

 * Returns true if the task does not share ->mm with another thread/process.

 */

bool is_single_threaded(struct task_struct *p)

bool is_single_threaded(struct task_struct *task)

{

	struct task_struct *g, *t;

	struct mm_struct *mm = p->mm;



	if (atomic_read(&p->signal->count) != 1)

		goto no;



	if (atomic_read(&p->mm->mm_users) != 1) {

		read_lock(&tasklist_lock);

		do_each_thread(g, t) {

			if (t->mm == mm && t != p)

				goto no_unlock;

		} while_each_thread(g, t);

		read_unlock(&tasklist_lock);

	}

	struct mm_struct *mm = task->mm;

	struct task_struct *p, *t;

	bool ret;



	return true;

	might_sleep();



no_unlock:

	read_unlock(&tasklist_lock);

no:

	if (atomic_read(&task->signal->live) != 1)

		return false;



	if (atomic_read(&mm->mm_users) == 1)

		return true;



	ret = false;

	down_write(&mm->mmap_sem);

	rcu_read_lock();

	for_each_process(p) {

		if (unlikely(p->flags & PF_KTHREAD))

			continue;

		if (unlikely(p == task->group_leader))

			continue;



		t = p;

		do {

			if (unlikely(t->mm == mm))

				goto found;

			if (likely(t->mm))

				break;

		} while_each_thread(p, t);

	}

	ret = true;

found:

	rcu_read_unlock();

	up_write(&mm->mmap_sem);



	return ret;

}