kernel: fix is_single_threaded (d2e3ee9b) · Commits · 戴 / test

- 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:

Oleg 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:

Andrew Morton <akpm@linux-foundation.org> Signed-off-by:

James Morris <jmorris@namei.org>

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;
		}

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