linux-kernel - Re: [PATCH 2/4] signal: Make flush_sigqueue() use free

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Date:   Fri, 22 Mar 2019 12:10:27 -0400
From:   Waiman Long <longman@...hat.com>
To:     Oleg Nesterov <oleg@...hat.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        selinux@...r.kernel.org, Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Eric Paris <eparis@...isplace.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: Re: [PATCH 2/4] signal: Make flush_sigqueue() use free_q to release
 memory

On 03/22/2019 07:16 AM, Oleg Nesterov wrote:
> 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.

Thanks for looking into that. As I am not knowledgeable enough about the
signal handling code path, I choose the lowest risk approach of not
trying to change the code flow while deferring memory deallocation after
releasing the tasklist_lock.

>> 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.
>
I am looking forward to it.

Thanks,
Longman