Messages in this thread Patch in this message | | | Date | Mon, 13 Apr 2009 23:45:13 +0200 | From | Oleg Nesterov <> | Subject | [PATCH] rework/fix is_single_threaded() |
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- 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>
--- 6.30/lib/is_single_threaded.c~ISS 2009-04-06 00:03:42.000000000 +0200 +++ 6.30/lib/is_single_threaded.c 2009-04-13 23:39:56.000000000 +0200 @@ -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; + struct mm_struct *mm = task->mm; + struct task_struct *p, *t; + bool ret; - if (atomic_read(&p->signal->count) != 1) - goto no; + might_sleep(); - 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); - } + if (atomic_read(&task->signal->live) != 1) + return false; - return true; + if (atomic_read(&mm->mm_users) == 1) + return true; -no_unlock: - read_unlock(&tasklist_lock); -no: - return false; + 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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