Re: [PATCH 1/3] mount.nfs: Refactor mount version and protocol autonegotiation

[Neil Brown <neilb@suse.de>]

On Thu, 9 Sep 2010 13:27:38 -0400
Chuck Lever <chuck.lever@oracle.com> wrote:

> 
> On Sep 6, 2010, at 9:12 PM, Neil Brown wrote:
> 
> > On Tue, 31 Aug 2010 17:21:31 -0400
> > Chuck Lever <chuck.lever@oracle.com> wrote:
> > 
> >> The logic that manages negotiating NFS version and protocol settings
> >> is getting more complex over time, so let's split this out of
> >> nfs_try_mount().
> >> 
> >> This should make Bruce a little happier, as it eliminates the silent
> >> switch case fall through in nfs_try_mount().  And it should help Neil
> >> fix some bugs he's found in this logic.
> > 
> > Hi Chuck,
> > thanks for these..
> > One question....
> > 
> > .....
> >> +{
> >> +	int result;
> >> +
> >> +	/*
> >> +	 * Before 2.6.32, the kernel NFS client didn't support
> >> +	 * "-t nfs vers=4" mounts, so NFS version 4 cannot be
> >> +	 * included when autonegotiating while running on
> >> +	 * those kernels.
> >> +	 */
> >> +	if (linux_version_code() <= MAKE_VERSION(2, 6, 31))
> >> +		goto fall_back;
> >> +
> >> +	errno = 0;
> >> +	result = nfs_try_mount_v4(mi);
> >> +	switch (errno) {
> > 
> > Should we not have e.g.
> > 
> >   if (result)
> >         return result;
> > 
> > before the switch(errno)??  Typically errno is 'undefined' in no error is
> > reported.
> > 
> > I realise the current code doesn't have that test either, but I still think
> > it is wrong not to.
> 
> We have
> 
>   errno = 0;
> 
> just before the nfs_try_mount_v4(mi); call.  This defines the value of errno even if nothing in nfs_try_mount_v4() sets it.

True, but what if something in nfs_try_mount_v4 does set it, but success is
finally returned?  That is certainly possible if mi has multiple addresses
and the first is unreachable.

I think you must *never* test errno unless the most recent call reported an
error.

> 
> Perhaps that violates Bruce's "too subtle" rule and I should replace it with your suggested post-call check?

He's like a guardian angel sitting on you shoulder whispering "too subtle"
all the time :-)  Definitely true this time.

Thanks,
NeilBrown


> 
> > Thanks
> > NeilBrown
> > 
> > 
> > 
> >> +	case EPROTONOSUPPORT:
> >> +		/* A clear indication that the server or our
> >> +		 * client does not support NFS version 4. */
> >> +		goto fall_back;
> >> +	case ENOENT:
> >> +		/* Legacy Linux servers don't export an NFS
> >> +		 * version 4 pseudoroot. */
> >> +		goto fall_back;
> >> +	case EPERM:
> >> +		/* Linux servers prior to 2.6.25 may return
> >> +		 * EPERM when NFS version 4 is not supported. */
> >> +		goto fall_back;
> >> +	default:
> >> +		return result;
> >> +	}
> >> +
> >> +fall_back:
> >> +	return nfs_try_mount_v3v2(mi);
> >> +}
> >> +
> >> +/*
> >>  * This is a single pass through the fg/bg loop.
> >>  *
> >>  * Returns TRUE if successful, otherwise FALSE.
> >> @@ -758,20 +801,8 @@ static int nfs_try_mount(struct nfsmount_info *mi)
> >> 
> >> 	switch (mi->version) {
> >> 	case 0:
> >> -		if (linux_version_code() > MAKE_VERSION(2, 6, 31)) {
> >> -			errno = 0;
> >> -			result = nfs_try_mount_v4(mi);
> >> -			if (errno != EPROTONOSUPPORT) {
> >> -				/* 
> >> -				 * To deal with legacy Linux servers that don't
> >> -				 * automatically export a pseudo root, retry
> >> -				 * ENOENT errors using version 3. And for
> >> -				 * Linux servers prior to 2.6.25, retry EPERM
> >> -				 */
> >> -				if (errno != ENOENT && errno != EPERM)
> >> -					break;
> >> -			}
> >> -		}
> >> +		result = nfs_autonegotiate(mi);
> >> +		break;
> >> 	case 2:
> >> 	case 3:
> >> 		result = nfs_try_mount_v3v2(mi);
> > 
>