69 lines
2.7 KiB
Plaintext
69 lines
2.7 KiB
Plaintext
File Locking Release Notes
|
|
|
|
Andy Walker <andy@lysaker.kvaerner.no>
|
|
|
|
12 May 1997
|
|
|
|
|
|
1. What's New?
|
|
--------------
|
|
|
|
1.1 Broken Flock Emulation
|
|
--------------------------
|
|
|
|
The old flock(2) emulation in the kernel was swapped for proper BSD
|
|
compatible flock(2) support in the 1.3.x series of kernels. With the
|
|
release of the 2.1.x kernel series, support for the old emulation has
|
|
been totally removed, so that we don't need to carry this baggage
|
|
forever.
|
|
|
|
This should not cause problems for anybody, since everybody using a
|
|
2.1.x kernel should have updated their C library to a suitable version
|
|
anyway (see the file "Documentation/process/changes.rst".)
|
|
|
|
1.2 Allow Mixed Locks Again
|
|
---------------------------
|
|
|
|
1.2.1 Typical Problems - Sendmail
|
|
---------------------------------
|
|
Because sendmail was unable to use the old flock() emulation, many sendmail
|
|
installations use fcntl() instead of flock(). This is true of Slackware 3.0
|
|
for example. This gave rise to some other subtle problems if sendmail was
|
|
configured to rebuild the alias file. Sendmail tried to lock the aliases.dir
|
|
file with fcntl() at the same time as the GDBM routines tried to lock this
|
|
file with flock(). With pre 1.3.96 kernels this could result in deadlocks that,
|
|
over time, or under a very heavy mail load, would eventually cause the kernel
|
|
to lock solid with deadlocked processes.
|
|
|
|
|
|
1.2.2 The Solution
|
|
------------------
|
|
The solution I have chosen, after much experimentation and discussion,
|
|
is to make flock() and fcntl() locks oblivious to each other. Both can
|
|
exists, and neither will have any effect on the other.
|
|
|
|
I wanted the two lock styles to be cooperative, but there were so many
|
|
race and deadlock conditions that the current solution was the only
|
|
practical one. It puts us in the same position as, for example, SunOS
|
|
4.1.x and several other commercial Unices. The only OS's that support
|
|
cooperative flock()/fcntl() are those that emulate flock() using
|
|
fcntl(), with all the problems that implies.
|
|
|
|
|
|
1.3 Mandatory Locking As A Mount Option
|
|
---------------------------------------
|
|
|
|
Mandatory locking, as described in
|
|
'Documentation/filesystems/mandatory-locking.txt' was prior to this release a
|
|
general configuration option that was valid for all mounted filesystems. This
|
|
had a number of inherent dangers, not the least of which was the ability to
|
|
freeze an NFS server by asking it to read a file for which a mandatory lock
|
|
existed.
|
|
|
|
From this release of the kernel, mandatory locking can be turned on and off
|
|
on a per-filesystem basis, using the mount options 'mand' and 'nomand'.
|
|
The default is to disallow mandatory locking. The intention is that
|
|
mandatory locking only be enabled on a local filesystem as the specific need
|
|
arises.
|
|
|