85 lines
3.4 KiB
Text
85 lines
3.4 KiB
Text
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File Locking Release Notes
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Andy Walker <andy@lysaker.kvaerner.no>
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12 May 1997
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1. What's New?
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--------------
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1.1 Broken Flock Emulation
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--------------------------
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The old flock(2) emulation in the kernel was swapped for proper BSD
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compatible flock(2) support in the 1.3.x series of kernels. With the
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release of the 2.1.x kernel series, support for the old emulation has
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been totally removed, so that we don't need to carry this baggage
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forever.
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This should not cause problems for anybody, since everybody using a
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2.1.x kernel should have updated their C library to a suitable version
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anyway (see the file "Documentation/Changes".)
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1.2 Allow Mixed Locks Again
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---------------------------
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1.2.1 Typical Problems - Sendmail
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---------------------------------
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Because sendmail was unable to use the old flock() emulation, many sendmail
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installations use fcntl() instead of flock(). This is true of Slackware 3.0
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for example. This gave rise to some other subtle problems if sendmail was
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configured to rebuild the alias file. Sendmail tried to lock the aliases.dir
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file with fcntl() at the same time as the GDBM routines tried to lock this
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file with flock(). With pre 1.3.96 kernels this could result in deadlocks that,
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over time, or under a very heavy mail load, would eventually cause the kernel
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to lock solid with deadlocked processes.
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1.2.2 The Solution
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------------------
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The solution I have chosen, after much experimentation and discussion,
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is to make flock() and fcntl() locks oblivious to each other. Both can
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exists, and neither will have any effect on the other.
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I wanted the two lock styles to be cooperative, but there were so many
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race and deadlock conditions that the current solution was the only
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practical one. It puts us in the same position as, for example, SunOS
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4.1.x and several other commercial Unices. The only OS's that support
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cooperative flock()/fcntl() are those that emulate flock() using
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fcntl(), with all the problems that implies.
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1.3 Mandatory Locking As A Mount Option
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---------------------------------------
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Mandatory locking, as described in 'Documentation/mandatory.txt' was prior
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to this release a general configuration option that was valid for all
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mounted filesystems. This had a number of inherent dangers, not the least
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of which was the ability to freeze an NFS server by asking it to read a
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file for which a mandatory lock existed.
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From this release of the kernel, mandatory locking can be turned on and off
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on a per-filesystem basis, using the mount options 'mand' and 'nomand'.
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The default is to disallow mandatory locking. The intention is that
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mandatory locking only be enabled on a local filesystem as the specific need
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arises.
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Until an updated version of mount(8) becomes available you may have to apply
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this patch to the mount sources (based on the version distributed with Rick
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Faith's util-linux-2.5 package):
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*** mount.c.orig Sat Jun 8 09:14:31 1996
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--- mount.c Sat Jun 8 09:13:02 1996
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***************
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*** 100,105 ****
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--- 100,107 ----
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{ "noauto", 0, MS_NOAUTO }, /* Can only be mounted explicitly */
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{ "user", 0, MS_USER }, /* Allow ordinary user to mount */
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{ "nouser", 1, MS_USER }, /* Forbid ordinary user to mount */
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+ { "mand", 0, MS_MANDLOCK }, /* Allow mandatory locks on this FS */
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+ { "nomand", 1, MS_MANDLOCK }, /* Forbid mandatory locks on this FS */
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/* add new options here */
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#ifdef MS_NOSUB
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{ "sub", 1, MS_NOSUB }, /* allow submounts */
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