Bug 396281

A closer looks shows that the 'cat' command is not
require.  Simply 'find'ing the files on the Windows
machine with

find . -type f -print >/dev/null

produces the behavior.  Also determined the problem
appears for both 'i686' and 'x86_64' servers, so it's
a portable bug.

Also it seems that more than a handful of files
is required to produce the problem.

Further details:

does not happen at top-level of NFSv3 mount, must be in subdirectory

if any directories are touched by the 'find' on Samba share, problem is suppressed

does not happen when files reside on 'ext2' file system

does happen when files reside on 'ext3' file system
  have 'noreservation' option active
  too lazy to try it with 'noreservation' turned off

tried mounting Samba share remotely with CIFS
  problem does not happen with 'find' run on CIFS mount

files accessed via Windows UNC path, not mapped drive

obviously CYGWIN is in use on the Windows system

can also produce the problem using CMD shell and 'type \\server\...\file' command
   CMD 'dir' doesn't work, probably because directory file is examined

does happen with even tiny number of files

does happen with WXP SP2 32-bit Samba client

can produce problem with MKS, but a 'cat' is also required:
  find . -type f -print | xargs cat >/dev/null

Created attachment 267991 [details]
TAR archive of traces files showing problem

Traces showing problem.  Debug level 10 Samba
trace plus Wireshark capture.

# Windows 'type' access of file

04:00:41
SMB access of 'djl0' at packet 21
    Last Write: Nov 24, 2007 02:20:44.000000000

# first 'find' invocation

04:00:45
NFS access of 'djl0' at packet 97
    mtime: Nov 24, 2007 04:00:45.000000000

04:00:45
NFS access of 'djl0' at packet 107
    mtime: Nov 24, 2007 04:00:45.000000000

# second 'find' invocation

04:00:54
NFS access of 'djl0' at packet 176
    mtime: Nov 24, 2007 02:20:44.000000000

Created attachment 268101 [details]
file lease test program

Examined Samba traces and discerned that the cause of the problem:

Samba issues

fcntl(fd, F_SETLEASE, F_WRLCK);

on files that are opened for read access.  The Linux kernel sets 
the modify time as seen via NFS as current when the activity 
trace is put in place and then restores the modify time once 
it's released.	Reproduced this with the attached test case.  The 
altered modify time appears *only* via NFS.  On the local system 
it's not visible.  Definitely a NFS bug.

Checked NFSv4 and issue is not present there.  Appears only with
NVSv3 and is in the server logic, not in the client side.

The Plot Thickens
=================

It turns out the observed behavior is by design; full details at

http://bugzilla.kernel.org/show_bug.cgi?id=9454

I don't fully understand all the interactions, so I'm pasting 
comments from Bruce Fields, a 'nfsd' maintainer.  Appended the
same information to the Samba bug report.

>------- Comment  #5 From bfields  2007-11-26 08:57:56  [reply] -------
>
>This is by design--see fs/locks.c:lease_get_mtime().
>
>The argument is: if somebody has a write lease on the file (are 
>you exporting this via Samba?  That'd be the typical user), then 
>they're caching writes--they're explicitly telling us that we do 
>not know whether the file is still the same, because we may have 
>modified it on a remote client and not told us about it.  So 
>lease_get_mtime() reports the current time as the mtime, 
>prompting you to actually try opening the file, at which point 
>the write lease gets broken and any cached writes get flushed 
>out.
>
>It's a terrible kludge, I agree, and maybe we should remove it.  
>But I'd first like to understand what circumstances prompted 
>somebody to add it originally, and talk to the Samba people 
>about how they're using these write leases.
>
>(In NFSv4, by the way, there's a callback to the client to allow 
>the server to find out the attributes of a file that the client 
>holding a write lease is caching, which solves this problem.  We 
>haven't implemented that yet; it seems likely to be an enormous 
>pain.  I wonder if Samba would want something similar?)
>
>
>------- Comment #6 From starlight 2007-11-26 10:17:08 [reply] -------
>
>Yes, using a Samba mount along with NFS mounts to 'make' build
>an application on multiple platforms simultaneously.  When
>re-building existing trees the result is spurious target 
>building, a real problem.  Took months of frustration to
>figure out what was going on.
>
>Actually reported this as a bug to Samba along with RH, and a
>request for info from the Samba team provoked the analysis that
>isolated the problem.  Closed the Samba bug but it's all still
>there:
>
>https://bugzilla.samba.org/show_bug.cgi?id=5103
>
>My impression from looking at the Samba code is that the leases
>are put in place so that files can be monitored for changes,
>but that's just a guess.  Perhaps Samba can use a read lease
>instead of a write lease.
>
>
>------- Comment #7 From bfields 2007-11-26 10:55:17 [reply] -------
>
>Thanks for the pointer to the samba bug.  I understand the 
>frustration.
>
>My feeling is that: yes, it's suboptimal (but perhaps not a bug) 
>for samba to be requesting a write lease when a read lease 
>(sufficient to alert it to modifications of the file) would be 
>enough.
>
>It seems to me that the real bug is the incomplete lease 
>implementation--if the purpose of a lease is to allow a remote 
>client to cache writes to the file, then there's no way for us 
>to give a sensible answer to a stat call, unless we break the 
>lease first.
>
>Perhaps we could find out (in the samba case) what the 
>consequences would be of not updating mtime in this case?  I 
>suppose the worst case would be that a modification to a file 
>made on a samba client could be indefinitely delayed from being 
>flushed to the server.
>
>If we agree that that would be less of a problem than these 
>spurious bumps in the mtime, then the best solution for now may 
>just be to rip out lease_get_mtime().  I'll cook up a straw-man 
>patch....
>
>
>------- Comment #8 From bfields 2007-11-26 10:56:16 [reply] -------
>
>Created an attachment (id=13763) [details]
>remove lease_get_mtime
>
>Not sure if this is what we want to do, but it's at least 
>something you could test to confirm the source of the problem.
>
>
>------- Comment #9 From starlight 2007-11-26 
>11:13:01 [reply] -------
>
>Thanks!  Happen to have a kernel source tree set up for the 
>RH images running on the server, so I can try this out in 
>the next week or so.  Seems certain to work.
>
>In the interim perhaps you could start a dialog with Samba about 
>the reasoning behind the application of the F_WRLCK?  Or if you 
>wish I could do so using the Samaba bug report cited above.
>

Created attachment 270671 [details]
kernel-2.6.9-55_remove_lease_get_mtime.patch

Adapted kernel.org authored patch for RHEL 4.5 2.6.9-55 kernel 
(attached).  Works as expected and eliminates spurious target 
rebuilding when a concurrent Samaba and NFS 'make' is run on 
same source tree. Test case also shows expected behavior.

Obviously did not perform any regression testing with other 
Samba file scenarios.  In our build trees the same files and 
directories are never written by more than one server.

Concise description of patch rationale from original:

>From: J. Bruce Fields <>
>Date: Mon, 26 Nov 2007 13:48:34 -0500
>Subject: [PATCH] nfsd: stop incrementing mtime on presence of write lease
>
>The lease_get_mtime() function has the effect of setting the mtime of a
>file (as far as any nfs client is concerned) to the current time,
>whenever there is a write lease held on the file.
>
>The presence of a write lease may mean that some client (almost
>certainly a samba client) is caching writes to that file.  Thus
>increasing the mtime has the effect of making the nfs client read data
>from that file, thus opening the file for read, thus breaking the write
>lease, thus forcing the samba client to flush any cached writes.
>
>However Samba seems to be requesting write leases even when a read lease
>would do.  And unfortunately the consequences of the spurious mtime
>updates--make unnecessarily rebuilding, for example--may be worse than
>the consequences of no mtime update--modifications from a samba client
>taking longer to be noticed on the nfs client.
>
>So perhaps we should rip out lease_get_mtime().

Hmmm...at first glance this sounds tricky and I'm not clear whether it's
something we'd want to take for RHEL4. I need to do a more thorough review of
the problem and patch though, so let me do that before I NAK anything...

This definitely needs to go upstream first and will need some soak time before
it can go into any RHEL product. Given that RHEL4 is approaching maintenance
phase, it's probably unlikely that this will ever make it into there. RHEL5
might be a possibility if there's resolution upstream.

Most likely, samba is getting leases in order to emulate opportunistic locks
(aka oplocks) on the share. One possible workaround might be to disable kernel
oplocks in smb.conf:

    kernel oplocks = no

...or maybe by judicious use of the "veto oplock files" parameter. Of course,
that means that oplocks/leases won't be broken by actvity on the files outside
of samba, but that's probably similar to the behavior you'll see by removing
lease_get_mtime anyway.

An even better option might be to remove NFS from the picture altogether and
just use CIFS with unix extensions on the Linux clients.

In any case, I don't see this happening for RHEL4 so I'm going to NAK this for
now. Feel free to open a similar case for RHEL5 if you like, though until this
patch goes upstream it's not an option there either...

Development Management has reviewed and declined this request.  You may appeal
this decision by reopening this request. 

Tried "kernel oplocks = no" in the Samba config
and it does work around the issue.