linux-kernel - Re: [GIT PULL] overlay filesystem v25

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Message-ID: <20141025081845.GJ7996@ZenIV.linux.org.uk>
Date:	Sat, 25 Oct 2014 09:18:45 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-unionfs@...r.kernel.org
Subject: Re: [GIT PULL] overlay filesystem v25

On Fri, Oct 24, 2014 at 09:24:45AM +0200, Miklos Szeredi wrote:

> The reason I didn't do your "fix" is that it
> 
>  - adds more lines than it takes,
> 
>  - I wasn't sure at all if the lockless access is actually correct
> without the ACCESS_ONCE and all the memory barrier magic that might be
>  necessary on weird architectures.

_What_ lockless accesses?  There is an extremely embarrassing bug in that
commit, all right, but it has nothing to do with barriers...  All
barrier-related issues are taken care of by ovl_path_upper() (and without
that you'd have tons of worse problems).  Fetching ->upperfile outside of
->i_mutex is fine - in the worst case we'll fetch NULL, open the sucker
grab ->i_mutex and find out that it has already been taken care of.
In which case we fput() what we'd opened and move on (fput() under
->i_mutex is fine - it's going to be delayed until return from syscall
anyway).

There was a very dumb braino in there; fixed, force-pushed, passes unionmount
tests, no regressions on LTP syscall ones and xfstests.
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