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Linux/Documentation/filesystems/locks.rst

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Diff markup

Differences between /Documentation/filesystems/locks.rst (Version linux-6.12-rc7) and /Documentation/filesystems/locks.rst (Version linux-6.9.12)


  1 .. SPDX-License-Identifier: GPL-2.0                 1 .. SPDX-License-Identifier: GPL-2.0
  2                                                     2 
  3 ==========================                          3 ==========================
  4 File Locking Release Notes                          4 File Locking Release Notes
  5 ==========================                          5 ==========================
  6                                                     6 
  7                 Andy Walker <andy@lysaker.kvaer      7                 Andy Walker <andy@lysaker.kvaerner.no>
  8                                                     8 
  9                             12 May 1997             9                             12 May 1997
 10                                                    10 
 11                                                    11 
 12 1. What's New?                                     12 1. What's New?
 13 ==============                                     13 ==============
 14                                                    14 
 15 1.1 Broken Flock Emulation                         15 1.1 Broken Flock Emulation
 16 --------------------------                         16 --------------------------
 17                                                    17 
 18 The old flock(2) emulation in the kernel was s     18 The old flock(2) emulation in the kernel was swapped for proper BSD
 19 compatible flock(2) support in the 1.3.x serie     19 compatible flock(2) support in the 1.3.x series of kernels. With the
 20 release of the 2.1.x kernel series, support fo     20 release of the 2.1.x kernel series, support for the old emulation has
 21 been totally removed, so that we don't need to     21 been totally removed, so that we don't need to carry this baggage
 22 forever.                                           22 forever.
 23                                                    23 
 24 This should not cause problems for anybody, si     24 This should not cause problems for anybody, since everybody using a
 25 2.1.x kernel should have updated their C libra     25 2.1.x kernel should have updated their C library to a suitable version
 26 anyway (see the file "Documentation/process/ch     26 anyway (see the file "Documentation/process/changes.rst".)
 27                                                    27 
 28 1.2 Allow Mixed Locks Again                        28 1.2 Allow Mixed Locks Again
 29 ---------------------------                        29 ---------------------------
 30                                                    30 
 31 1.2.1 Typical Problems - Sendmail                  31 1.2.1 Typical Problems - Sendmail
 32 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                  32 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 33 Because sendmail was unable to use the old flo     33 Because sendmail was unable to use the old flock() emulation, many sendmail
 34 installations use fcntl() instead of flock().      34 installations use fcntl() instead of flock(). This is true of Slackware 3.0
 35 for example. This gave rise to some other subt     35 for example. This gave rise to some other subtle problems if sendmail was
 36 configured to rebuild the alias file. Sendmail     36 configured to rebuild the alias file. Sendmail tried to lock the aliases.dir
 37 file with fcntl() at the same time as the GDBM     37 file with fcntl() at the same time as the GDBM routines tried to lock this
 38 file with flock(). With pre 1.3.96 kernels thi     38 file with flock(). With pre 1.3.96 kernels this could result in deadlocks that,
 39 over time, or under a very heavy mail load, wo     39 over time, or under a very heavy mail load, would eventually cause the kernel
 40 to lock solid with deadlocked processes.           40 to lock solid with deadlocked processes.
 41                                                    41 
 42                                                    42 
 43 1.2.2 The Solution                                 43 1.2.2 The Solution
 44 ^^^^^^^^^^^^^^^^^^                                 44 ^^^^^^^^^^^^^^^^^^
 45 The solution I have chosen, after much experim     45 The solution I have chosen, after much experimentation and discussion,
 46 is to make flock() and fcntl() locks oblivious     46 is to make flock() and fcntl() locks oblivious to each other. Both can
 47 exists, and neither will have any effect on th     47 exists, and neither will have any effect on the other.
 48                                                    48 
 49 I wanted the two lock styles to be cooperative     49 I wanted the two lock styles to be cooperative, but there were so many
 50 race and deadlock conditions that the current      50 race and deadlock conditions that the current solution was the only
 51 practical one. It puts us in the same position     51 practical one. It puts us in the same position as, for example, SunOS
 52 4.1.x and several other commercial Unices. The     52 4.1.x and several other commercial Unices. The only OS's that support
 53 cooperative flock()/fcntl() are those that emu     53 cooperative flock()/fcntl() are those that emulate flock() using
 54 fcntl(), with all the problems that implies.       54 fcntl(), with all the problems that implies.
 55                                                    55 
 56                                                    56 
 57 1.3 Mandatory Locking As A Mount Option            57 1.3 Mandatory Locking As A Mount Option
 58 ---------------------------------------            58 ---------------------------------------
 59                                                    59 
 60 Mandatory locking was prior to this release a      60 Mandatory locking was prior to this release a general configuration option
 61 that was valid for all mounted filesystems.  T     61 that was valid for all mounted filesystems.  This had a number of inherent
 62 dangers, not the least of which was the abilit     62 dangers, not the least of which was the ability to freeze an NFS server by
 63 asking it to read a file for which a mandatory     63 asking it to read a file for which a mandatory lock existed.
 64                                                    64 
 65 Such option was dropped in Kernel v5.14.           65 Such option was dropped in Kernel v5.14.
                                                      

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