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SubjectRe: [PATCH -mm 0/3] proc: task->signal can't be NULL
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Oleg Nesterov <oleg@redhat.com> writes:

> With the recent changes in -mm it is always safe to dereference
> task->signal. It can't be NULL and it is pinned to task_struct.
>
> fs/proc becomes the only valid user of signal->count which should
> either die or become "int nr_threads".
>
>
> Alexey, Eric.
>
> Can't we kill this counter? Afaics, get_nr_threads() doesn't need to
> be "precise", we probably can estimate the number of threads using
> signal->live (yes sure, we can't use ->live as nr_threads).
>
> Except: first_tid() uses get_nr_threads() for optimization. Is this
> optimization really important? Afaics, it only helps in the unlikely
> case, probably in that case the extra lockless while_each_thread()
> doesn't hurt.
>
> IOW, how about
>
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -3071,11 +3071,6 @@ static struct task_struct *first_tid(str
> goto found;
> }
>
> - /* If nr exceeds the number of threads there is nothing todo */
> - pos = NULL;
> - if (nr && nr >= get_nr_threads(leader))
> - goto out;
> -
> /* If we haven't found our starting place yet start
> * with the leader and walk nr threads forward.
> */
>
> ?
>
> Not that I think it is terribly important to kill this counter, and
> probably signal->nr_threads can make sense anyway, so far I am just
> curious.

I think that was just a sanity check since it was easy. I want to say
it prevents a DOS attack with user space passing unreasonably large
file position but that DOS attack is handled by ensuring we don't walk
through the list if threads more than once.

However:
proc_task_getattr uses get_nr_threads to get it's nlink count correct.

Not walking the thread list to get the number of threads seems like an
important cpu time saving measure.

Eric



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