linux-kernel - Re: [PATCH 0/5] RCU-walk support for autofs

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Date:	Tue, 19 Aug 2014 21:16:54 +1000
From:	NeilBrown <neilb@...e.de>
To:	Ian Kent <raven@...maw.net>
Cc:	autofs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/5] RCU-walk support for autofs

On Tue, 19 Aug 2014 18:02:27 +0800 Ian Kent <raven@...maw.net> wrote:

> On Mon, 2014-08-18 at 16:25 +0800, Ian Kent wrote:
> > On Mon, 2014-08-18 at 16:33 +1000, NeilBrown wrote:
> > > Hi Ian,
> > >  Have you had a chance to run your tests in these patches yet?
> > >  I've done what testing I can think of and cannot fault them.
> > 
> > I haven't, I've been plagued with illness so I'm not getting nearly
> > enough done. I'll try to put a kernel together and run the test in the
> > next week or so.
> 
> Just to let you know that I managed to spend some time on this. I built
> a kernel (3.17.0-rc1) with the series and ran a couple of tests.
> 
> I'm not certain that the patches I used are identical to your posting, I
> saw one difference, after the fact, that shouldn't matter, I'll have to
> check that.
> 
> It isn't possible to test expire to mount races because the mounts in
> the tree never expire.
> 
> At first I thought it was because so many processes were accessing the
> tree all the time but manually constructing the maps and mounting the
> mounts shows that nothing ever expires, at least for this tree.
> 
> However, issuing a shut down does expire all the mounts and shuts down
> autofs cleanly.
> 
> So there is something not quite right with the expire check or my
> patches have mistakes.

Ahh.. I bet I know what it is.
autofs4_can_expire() isn't idempotent.
Because we call should_expire twice, autofs4_can_expire() is called twice and
the second time it failed because  the first time it resets ->last_used.

diff --git a/fs/autofs4/expire.c b/fs/autofs4/expire.c
index eb4b770a4bf6..80133a9d9427 100644
--- a/fs/autofs4/expire.c
+++ b/fs/autofs4/expire.c
@@ -26,6 +26,9 @@ static inline int autofs4_can_expire(struct dentry *dentry,
 	if (ino == NULL)
 		return 0;
 
+	if (ino->flags & AUTOFS_INF_NO_RCU)
+		/* Already performed this test */
+		return 1;
 	if (!do_now) {
 		/* Too young to die */
 		if (!timeout || time_after(ino->last_used + timeout, now))



might fix it, but might be a hack.
(tests.... yep, that seems to fix it).

I'll think some more tomorrow.

Thanks,
NeilBrown

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