linux-kernel - Re: Dcache oops

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Message-ID: <20160603222355.GW14480@ZenIV.linux.org.uk>
Date:	Fri, 3 Jun 2016 23:23:55 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Oleg Drokin <green@...uxhacker.ru>,
	"<linux-kernel@...r.kernel.org> Mailing List" 
	<linux-kernel@...r.kernel.org>,
	"<linux-fsdevel@...r.kernel.org>" <linux-fsdevel@...r.kernel.org>
Subject: Re: Dcache oops

On Fri, Jun 03, 2016 at 03:00:02PM -0700, Linus Torvalds wrote:

> Is perhaps the "delayed_call" logic broken, and the symlink is free'd too early?
> 
> That whole set_delayed_call/do_delayed_call thing came in 4.5. Maybe
> something broke that logic, and we've executed the delayed freeing
> before we should have.
> 
> Normally it's done at terminate_walk() time. But I note that in
> walk_component(), we do put_link(nd) which does a do_delayed_call(),
> but does *not* do a clear_delayed_call(), so now I think a subsequent
> terminate_walk() might drop it *again*.

Nope - put_link() also decrements nd->depth.  No double calls there...

> I'm probably missing something, but I have to say that the old
> explicit "put_link()" callback logic was more obvious than the new
> delayed calls are.

It's not that.  It's explicit put_link() in do_last(), followed by
ESTALEOPEN and subsequent misbegotten "retry the last step on ESTALEOPEN"
looking at now-freed nd->last.name.  IOW, the bug predates delayed_call
stuff.