patch-2.0.31 linux/Documentation/locks.txt

• Lines: 125
• Date: Sun Aug 3 13:59:07 1997
• Orig file: v2.0.30/linux/Documentation/locks.txt
• Orig date: Wed May 15 01:22:04 1996

diff -u --recursive --new-file v2.0.30/linux/Documentation/locks.txt linux/Documentation/locks.txt
@@ -2,37 +2,30 @@
 
 		Andy Walker <andy@lysaker.kvaerner.no>
 
-			    15 May 1996
+			    12 May 1997
 
 
-What's New?
------------
+1. What's New?
+--------------
 
-Flock Emulation Warnings
-------------------------
-Many people will have noticed the ugly messages that the file locking
-code started generating with the release of kernel version 1.3.95. The
-messages look something like this:
+1.1 Broken Flock Emulation
+--------------------------
 
-    fcntl_setlk() called by process XX with 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 is a warning for people using older C libraries that those libraries
-are still calling the pre 1.3.x flock() emulation routines, instead of
-the real flock() system call. The old routines are quite badly broken,
-especially with respect to parent-child lock sharing, and can give bad
-results if, for example, sendmail attempts to use them.
+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 "linux/Documentation/Changes".)
 
-Fixed versions of the C libraries have been on public release for many
-months. The latest versions are 5.2.18 or 5.3.12 for ELF, and I believe
-somebody made a 4.7.6 release for people using a.out systems.
+1.2 Allow Mixed Locks Again
+---------------------------
 
-In 1.3.96 Linus decided to be lenient on the stragglers and changed the
-warning message so that the kernel will only complain five times and
-then shut up. That should make life more bearable even for people who,
-for some reason, don't want to upgrade.
-
-Sendmail Problems
------------------
+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
@@ -42,23 +35,50 @@
 over time, or under a very heavy mail load, would eventually cause the kernel
 to lock solid with deadlocked processes.
 
-Disallow Mixed Locks
---------------------
-I have chosen the rather cruel solution of disallowing mixed locking styles
-on a given file at a given time. Attempts to lock a file with flock() when
-fcntl() locks exist, or vice versa, return with an error status of EBUSY.
-This seemed to be the only way to avoid all possible deadlock conditions,
-as flock() locks do not strictly have one owner process and so can't be
-checked for deadlocking in the usual manner.
-
-The process that created a lock with flock() might have forked multiple
-children and exited. Previously the parent process would have been marked
-as the owner of the lock, but deadlocks could just have easily occurred in
-one or more of the children, which we would not have been able to identify
-and avoid.
-
-Some programs may break (again, groan). In particular the aforementioned
-sendmail may have problems running in 'newaliases' mode. It will no longer
-deadlock though. Recompile sendmail to use flock() and your troubles will
-be over.
 
+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 serveral 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/mandatory.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.
+
+Until an updated version of mount(8) becomes available you may have to apply
+this patch to the mount sources (based on the version distributed with Rick
+Faiths util-linux-2.5 package):
+
+*** mount.c.orig	Sat Jun  8 09:14:31 1996
+--- mount.c	Sat Jun  8 09:13:02 1996
+***************
+*** 100,105 ****
+--- 100,107 ----
+    { "noauto",	0, MS_NOAUTO	},	/* Can  only be mounted explicitly */
+    { "user",	0, MS_USER	},	/* Allow ordinary user to mount */
+    { "nouser",	1, MS_USER	},	/* Forbid ordinary user to mount */
++   { "mand",	0, MS_MANDLOCK	},	/* Allow mandatory locks on this FS */
++   { "nomand",	1, MS_MANDLOCK	},	/* Forbid mandatory locks on this FS */
+    /* add new options here */
+  #ifdef MS_NOSUB
+    { "sub",	1, MS_NOSUB	},	/* allow submounts */