Status of fcntl() mandatory locking

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hello all,

For a long time now, the fcntl(2) man page has carried this text
regarding mandatory (byte-range) locking:


   Mandatory locking
       The  implementation  of mandatory locking in all known versions
       of Linux is subject to race conditions which render it  unreli‐
       able: a write(2) call that overlaps with a lock may modify data
       after the mandatory lock is acquired; a read(2) call that over‐
       laps with a lock may detect changes to data that were made only
       after a write lock was acquired.  Similar races  exist  between
       mandatory  locks  and  mmap(2).  It is therefore inadvisable to
       rely on mandatory locking.

I wanted to check: does it remain true with modern kernels that mandatory
locking is unreliable? If things have changed, an uopdate to the man page
is obviously in order.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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