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1.. SPDX-License-Identifier: GPL-2.02 3==========================4File Locking Release Notes5==========================6 7 Andy Walker <andy@lysaker.kvaerner.no>8 9 12 May 199710 11 121. What's New?13==============14 151.1 Broken Flock Emulation16--------------------------17 18The old flock(2) emulation in the kernel was swapped for proper BSD19compatible flock(2) support in the 1.3.x series of kernels. With the20release of the 2.1.x kernel series, support for the old emulation has21been totally removed, so that we don't need to carry this baggage22forever.23 24This should not cause problems for anybody, since everybody using a252.1.x kernel should have updated their C library to a suitable version26anyway (see the file "Documentation/process/changes.rst".)27 281.2 Allow Mixed Locks Again29---------------------------30 311.2.1 Typical Problems - Sendmail32^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^33Because sendmail was unable to use the old flock() emulation, many sendmail34installations use fcntl() instead of flock(). This is true of Slackware 3.035for example. This gave rise to some other subtle problems if sendmail was36configured to rebuild the alias file. Sendmail tried to lock the aliases.dir37file with fcntl() at the same time as the GDBM routines tried to lock this38file with flock(). With pre 1.3.96 kernels this could result in deadlocks that,39over time, or under a very heavy mail load, would eventually cause the kernel40to lock solid with deadlocked processes.41 42 431.2.2 The Solution44^^^^^^^^^^^^^^^^^^45The solution I have chosen, after much experimentation and discussion,46is to make flock() and fcntl() locks oblivious to each other. Both can47exists, and neither will have any effect on the other.48 49I wanted the two lock styles to be cooperative, but there were so many50race and deadlock conditions that the current solution was the only51practical one. It puts us in the same position as, for example, SunOS524.1.x and several other commercial Unices. The only OS's that support53cooperative flock()/fcntl() are those that emulate flock() using54fcntl(), with all the problems that implies.55 56 571.3 Mandatory Locking As A Mount Option58---------------------------------------59 60Mandatory locking was prior to this release a general configuration option61that was valid for all mounted filesystems. This had a number of inherent62dangers, not the least of which was the ability to freeze an NFS server by63asking it to read a file for which a mandatory lock existed.64 65Such option was dropped in Kernel v5.14.66