[man-pages][PATCH v1] flock.2: add CIFS details

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From: Aurelien Aptel <aaptel@xxxxxxxx>

Similarly to NFS, CIFS flock() locks behave differently than the
standard. Document those differences.

Signed-off-by: Aurelien Aptel <aaptel@xxxxxxxx>
---
 man2/flock.2 | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/man2/flock.2 b/man2/flock.2
index 61d4b5396..9271b8fef 100644
--- a/man2/flock.2
+++ b/man2/flock.2
@@ -239,6 +239,28 @@ see the discussion of the
 .I "local_lock"
 option in
 .BR nfs (5).
+.SS CIFS details
+CIFS mounts share similar limitations with NFS.
+.PP
+In Linux kernels up to 5.4,
+.BR flock ()
+locks files on the local system,
+not over SMB. A locked file won't appear locked for other SMB clients
+accessing the same share.
+.PP
+Since Linux 5.5,
+.BR flock ()
+are emulated with SMB byte-range locks on the
+entire file. Similarly to NFS, this means that
+.BR fcntl (2)
+and
+.BR flock ()
+locks interact with one another over SMB. Another important
+side-effect is that the locks are not advisory anymore: a write on a
+locked file will always fail with
+.BR EACCESS .
+This difference originates from the design of locks in the SMB
+protocol and cannot be worked around.
 .SH SEE ALSO
 .BR flock (1),
 .BR close (2),
-- 
2.30.0





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