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

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Message-ID: <CAJfpegvM8d-cizf=3KmTSFVgAYPBDp_0fQAh+G2Q87u=taXm3A@mail.gmail.com>
Date:	Sat, 25 Oct 2014 11:53:52 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Al Viro <viro@...iv.linux.org.uk>
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 Sat, Oct 25, 2014 at 10:18 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
> 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).

Yes, but it's not about race with copy-up (which the ovl_path_upper()
protects against), but race of two fsync calls with each other.  If
there's no synchronization between them, then that od->upperfile does
indeed count as lockless access, no matter that the assignment was
done under lock.

Thanks,
Miklos
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