linux-kernel - Re: [PATCH 0/1] autofs: fix memory leak of waitqueues in autofs_catatonic

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Date:   Fri, 10 Mar 2023 20:56:27 +0300
From:   Fedor Pchelkin <pchelkin@...ras.ru>
To:     Ian Kent <raven@...maw.net>, Al Viro <viro@...IV.linux.org.uk>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrei Vagin <avagin@...il.com>,
        Takeshi Misawa <jeliantsurux@...il.com>,
        autofs@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alexey Khoroshilov <khoroshilov@...ras.ru>,
        lvc-project@...uxtesting.org
Subject: Re: [PATCH 0/1] autofs: fix memory leak of waitqueues in
 autofs_catatonic_mode

On Mon, Feb 13, 2023 at 12:37:16PM +0800, Ian Kent wrote:
> 
> I was going to Ack the patch but I wondering if we should wait a little
> 
> while and perhaps (probably) include the wake up call change as well.
>

Hmm, those would be separate patches?

An interesting thing is that the code itself supposes the wake up calls
from autofs_wait_release() and autofs_catatonic_mode() to be related in
some way (see autofs_wait fragment):

	/*
	 * wq->name.name is NULL iff the lock is already released
	 * or the mount has been made catatonic.
	 */
	wait_event_killable(wq->queue, wq->name.name == NULL);
	status = wq->status;

It seems 'the lock is already released' refers to autofs_wait_release()
as there is no alternative except the call to catatonic function where
wq->name.name is NULL. So apparently the wake up calls should be the same
(although I don't know if autofs_catatonic_mode has some different
behaviour in such case, but probably it doesn't differ here).

It's also strange that autofs_kill_sb() calls autofs_catatonic_mode() and
currently it just decrements the wait_ctr's and it is not clear to me
where the waitqueues are eventually freed in such case. Only if
autofs_wait_release() or autofs_wait() are called? I'm not sure whether
they are definitely called after that or not.

[1] https://www.spinics.net/lists/autofs/msg01878.html
> 
> In any case we need Al to accept it (cc'd).
> 
> Hopefully Al will offer his opinion on the changes too.
> 

It would be very nice if probably Al would make it more clear.

At the moment I think that the leak issue should be fixed with the
currenly discussed patch and the wake up call issue should be fixed like
in [1], but perhaps I'm missing something.