[PATCH] flock.2: describe flock on NFS filesystems

[Petr Uzel <petr.uzel-AlSwsSmVLrQ@public.gmane.org>]

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NFSv4 need file to be opened in RW-mode to place exclusive lock:
http://www.spinics.net/lists/linux-nfs/msg18502.html

flock works on NFS since 2.6.12:
http://nfs.sourceforge.net/#section_d ,section D10

Signed-off-by: Petr Uzel <petr.uzel-AlSwsSmVLrQ@public.gmane.org>
---
 man2/flock.2 |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/man2/flock.2 b/man2/flock.2
index f0d85af..c904f6b 100644
--- a/man2/flock.2
+++ b/man2/flock.2
@@ -107,7 +107,8 @@ are preserved across an
 .BR execve (2).
 
 A shared or exclusive lock can be placed on a file regardless of the
-mode in which the file was opened.
+mode in which the file was opened. The exception is NFSv4, where
+the file has to be opened for writing to place exclusive lock.
 .SH "RETURN VALUE"
 On success, zero is returned.
 On error, \-1 is returned, and
@@ -146,7 +147,8 @@ possibly implemented in terms of
 appears on most UNIX systems.
 .SH NOTES
 .BR flock ()
-does not lock files over NFS.
+does not lock files over NFS before kernel 2.6.12. Since
+2.6.12, kernel emulates flock () using fcntl(2).
 Use
 .BR fcntl (2)
 instead: that does work over NFS, given a sufficiently recent version of
-- 
1.7.3.4


Petr

--
Petr Uzel
IRC: ptr_uzl @ freenode

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