Re: [PATCH] rework/fix is_single_threaded()

[Oleg Nesterov <oleg@redhat.com>]

On 06/18, Andrew Morton wrote:
>
> It appears that this patch is rather stuck.  Should I drop it?

Well, I am biased of course...

I think the patch is correct. David dislikes down_write(->mmap_sem),
but imho it is better than the global tasklist_lock.

Looks like we can avoid ->mmap_sem too, but imho this change needs
another patch, it is subtle.

OTOH, I can't say this patch is important, both the fixed bugs and
improvements are minor.

Oleg.

> From: Oleg Nesterov <oleg@redhat.com>
>
> - 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>
> ---
>
>  lib/is_single_threaded.c |   62 +++++++++++++++++++++----------------
>  1 file changed, 36 insertions(+), 26 deletions(-)
>
> diff -puN lib/is_single_threaded.c~rework-fix-is_single_threaded lib/is_single_threaded.c
> --- a/lib/is_single_threaded.c~rework-fix-is_single_threaded
> +++ a/lib/is_single_threaded.c
> @@ -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;
> +
> +	might_sleep();
> +
> +	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 true;
> -
> -no_unlock:
> -	read_unlock(&tasklist_lock);
> -no:
> -	return false;
> +	return ret;
>  }
> _
>