Re: [PATCH 2/4] signal: Make flush_sigqueue() use free_q to release memory

[Oleg Nesterov <oleg@redhat.com>]

On 03/21, Matthew Wilcox wrote:
>
> On Thu, Mar 21, 2019 at 05:45:10PM -0400, Waiman Long wrote:
>
> > To avoid this dire condition and reduce lock hold time of tasklist_lock,
> > flush_sigqueue() is modified to pass in a freeing queue pointer so that
> > the actual freeing of memory objects can be deferred until after the
> > tasklist_lock is released and irq re-enabled.
>
> I think this is a really bad solution.  It looks kind of generic,
> but isn't.  It's terribly inefficient, and all it's really doing is
> deferring the debugging code until we've re-enabled interrupts.

Agreed.

> We'd be much better off just having a list_head in the caller
> and list_splice() the queue->list onto that caller.  Then call
> __sigqueue_free() for each signal on the queue.

This won't work, note the comment which explains the race with sigqueue_free().

Let me think about it... at least we can do something like

	close_the_race_with_sigqueue_free(struct sigpending *queue)
	{
		struct sigqueue *q, *t;

		list_for_each_entry_safe(q, t, ...) {
			if (q->flags & SIGQUEUE_PREALLOC)
				list_del_init(&q->list);
	}

called with ->siglock held, tasklist_lock is not needed.

After that flush_sigqueue() can be called lockless in release_task() release_task.

I'll try to make the patch tomorrow.

Oleg.