Re: [GIT PULL] overlay filesystem v25

From: Miklos Szeredi
Date: Sat Oct 25 2014 - 15:17:33 EST


On Sat, Oct 25, 2014 at 10:18 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/