Re: [PATCH 0/5] fuse: handle release synchronously (v4)

[Maxim Patlasov <mpatlasov@parallels.com>]

Hi Miklos,

On 10/09/2014 12:14 PM, Miklos Szeredi wrote:
> On Wed, Oct 1, 2014 at 1:28 PM, Maxim Patlasov <mpatlasov@parallels.com> wrote:
>> Given those patches must die, do you have any ideas how to resolve that
>> "spurious EBUSY" problem?
> Check the "sync_release" branch of fuse:
>
>    git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git sync_release
>
> And same branch name for libfuse:
>
>    git://git.code.sf.net/p/fuse/fuse sync_release
>
> What it does is send RELEASE from ->flush() after checking the
> refcount of file (being careful about RCU accesses).
>
> Lightly tested, more testing, as well as review, is welcome.

Thank you very much for efforts, highly appreciated! I've had a close 
look at your patches and found a few issues. Most of them can be easily 
fixed, but one puzzles me: the way how you detect last flush is not race 
free. Something as simple as:

int main(int argc, char *argv[])
{
     int fd = open(argv[1], O_RDWR);
     fork();
}

may easily dive into fuse_try_sync_release() concurrently and both 
observe file->f_count == 2. Then both return falling back to sending the 
release asynchronously. This makes sync/async behaviour unpredictable 
even for well-behaved applications which don't do any esoteric things 
like racing i/o with close or exiting while a descriptor is in-flight in 
a unix domain socket.

I cannot see any way to recognise last flush without help of VFS layer, 
can you?

Thanks,
Maxim