Re: EACCES race when opening just-created file

[Noah Misch <noah@leadboat.com>]

On Sat, Jul 23, 2016 at 04:50:57PM -0400, Jeff Layton wrote:
> On Sat, 2016-07-23 at 16:12 -0400, Noah Misch wrote:
> > With the Linux kernel as both NFS client and server, I'm seeing a race
> > condition where open() can return EACCES for a just-created file.  Is this
> > expected behavior?  POSIX implicitly allows it, but I failed to locate any
> > discussion of it.  I have attached a test program; here, it witnesses the race
> > for roughly a dozen of the 4096 files it creates (different files each time).

> This is due to a limitation of NFSv3 and NFSv4.0. When you do an
> exclusive create, the atime and mtime get set to a particular set of
> values (the verifier), and the client is expected to override those
> values with a follow-on SETATTR call once it gets those values back.
> This is to prevent problems during certain server reboot scenarios. 
> 
> The NFS server also sets the mode to 0000 during this time (since the
> client can't even set the mode during the operation).

Interesting; that fully explains it.  Thanks.

> NFSv4.1 is less subject to this problem. You may want to try using that
> and see if it's any better.

I will look into that.