bin/111101: /usr/bin/lockf: when lockf blocks due to another lockf and no -k is specified and the other lockf ends, the file is away

[Arne Woerner]

>Number:         111101
>Category:       bin
>Synopsis:       /usr/bin/lockf: when lockf blocks due to another lockf and no -k is specified and the other lockf ends, the file is away
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Sun Apr 01 18:50:04 GMT 2007
>Closed-Date:
>Last-Modified:
>Originator:     Arne Woerner
>Release:        R6.2
>Organization:
>Environment:
FreeBSD neo.riddick.homeunix.org 6.2-RELEASE FreeBSD 6.2-RELEASE #8: Fri Mar 30 10:00:56 UTC 2007     aw at neo.riddick.homeunix.org:/usr/src/sys/i386/compile/NEO  i386

>Description:
I use lockf in order to coordinate the access to my TV frame grabbers (/dev/cxm?).

When another recording starts (process B) it can happen that a previous recording (process A) still needs some seconds, I end up with an unlink-ed lock file.

I would expect the following order:

1. process A: holds file LCK
2. process B: blocks on file LCK
3. process A: unlinks file LCK
4. process A: releases lock on file LCK
5. process B: locks the already opened but unlinked file LCK
6. process B: checks, if file LCK still exists
7. process B: creates file LCK
8. process B: locks file LCK

As a workaround I use lockf with option "-k", which should solve that problem...

Furthermore I would like to recommend, to make the "-k" behaviour the default behaviour, so that we will not remove data files, that were co-used as lock files...

-Arne
>How-To-Repeat:
Let LCK be a file, that does not exist in the work directory
% lockf LCK sleep 10 & sleep 1 ; lockf LCK ls -l LCK
[1] 38788
ls: LCK: No such file or directory
[1]  + Done                          lockf LCK sleep 10

I was able to repeat that on a i586 (Pentium) and a i686 (Athlon XP), which both used R6.2 with multimedia/pvr250 related kernel patches.
>Fix:
1.
Use "-k" as a work around.

2.
Check if file exists after holding the lock. And if it doesnt exist anymore, re-create it and close the old (in the meantime unlinked) lock file and start again... <-- Maybe that contains another error, that I dont see yet...
>Release-Note:
>Audit-Trail:
>Unformatted: