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From f1121d1e7ff76ace6e89fbe275abc3abedd5940b Mon Sep 17 00:00:00 2001
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From: Jan Chaloupka <jchaloup@redhat.com>
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Date: Mon, 29 Sep 2014 21:43:53 +0200
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Subject: [PATCH] flock.2 NFS flock
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---
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man-pages/man2/flock.2 | 48 ++++++++++++++++++++++++++++++++++++++++--------
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1 file changed, 40 insertions(+), 8 deletions(-)
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diff --git a/man-pages/man2/flock.2 b/man-pages/man2/flock.2
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index 60537fe..b8c55d1 100644
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--- a/man-pages/man2/flock.2
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+++ b/man-pages/man2/flock.2
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@@ -145,19 +145,12 @@ possibly implemented in terms of
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.BR fcntl (2),
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appears on most UNIX systems.
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.SH NOTES
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-.BR flock ()
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-does not lock files over NFS.
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-Use
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-.BR fcntl (2)
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-instead: that does work over NFS, given a sufficiently recent version of
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-Linux and a server which supports locking.
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-.PP
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Since kernel 2.0,
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.BR flock ()
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is implemented as a system call in its own right rather
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than being emulated in the GNU C library as a call to
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.BR fcntl (2).
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-This yields true BSD semantics:
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+This yields classical BSD semantics:
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there is no interaction between the types of lock
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placed by
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.BR flock ()
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@@ -166,6 +159,45 @@ and
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and
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.BR flock ()
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does not detect deadlock.
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+(Note, however, that on some modern BSDs,
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+.\" E.g., according to the flock(2) man page, FreeBSD since at least 5.3
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+.BR flock ()
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+and
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+.BR fcntl (2)
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+locks
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+.I do
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+interact with one another.)
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+.PP
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+In Linux kernels up to 2.6.11,
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+.BR flock ()
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+does not lock files over NFS
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+(i.e., the scope of locks was limited to the local system).
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+Instead, one could use
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+.BR fcntl (2)
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+byte-range locking, which does work over NFS,
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+given a sufficiently recent version of
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+Linux and a server which supports locking.
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+Since Linux 2.6.12, NFS clients support
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+.BR flock ()
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+locks by emulating them as byte-range locks on the entire file.
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+This means that
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+.BR fcntl (2)
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+and
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+.BR flock ()
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+locks
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+.I do
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+interact with one another over NFS.
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+Since Linux 2.6.37,
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+.\" commit 5eebde23223aeb0ad2d9e3be6590ff8bbfab0fc2
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+the kernel supports a compatibility mode that allows
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+.BR flock ()
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+locks (and also
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+.BR fcntl (2)
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+byte region locks) to be treated as local;
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+see the discussion of the
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+.I "local_lock"
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+option in
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+.BR nfs (5).
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.PP
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.BR flock ()
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places advisory locks only; given suitable permissions on a file,
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--
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1.9.3
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