[PATCH v4] flock.2: add CIFS details

["Aurélien Aptel" <aaptel@suse.com>]

From: Aurelien Aptel <aaptel@suse.com>

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

Here is the rendered text:

CIFS details
  In  Linux kernels up to 5.4, flock() is not propagated over SMB. A file
  with such locks will not appear locked for remote clients.

  Since Linux 5.5, flock() locks are emulated with SMB  byte-range  locks
  on  the  entire  file.  Similarly  to NFS, this means that fcntl(2) and
  flock() locks interact with one another. Another important  side-effect
  is  that  the  locks are not advisory anymore: a write on a locked file
  will always fail with EACCES.   This difference originates from the de-
  sign of locks in the SMB protocol, which provides mandatory locking se-
  mantics. The nobrl mount option (see mount.cifs(8)) turns off  fnctl(2)
  and  flock() lock propagation to remote clients and makes flock() locks
  advisory again.

Signed-off-by: Aurelien Aptel <aaptel@suse.com>
Reviewed-By: Tom Talpey <tom@talpey.com>
---
 man2/flock.2 | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/man2/flock.2 b/man2/flock.2
index 61d4b5396..d447eac9c 100644
--- a/man2/flock.2
+++ b/man2/flock.2
@@ -239,6 +239,35 @@ see the discussion of the
 .I "local_lock"
 option in
 .BR nfs (5).
+.SS CIFS details
+In Linux kernels up to 5.4,
+.BR flock ()
+is not propagated over SMB. A file with such locks will not appear
+locked for remote clients.
+.PP
+Since Linux 5.5,
+.BR flock ()
+locks 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. Another important side-effect is that
+the locks are not advisory anymore: a write on a locked file will
+always fail with
+.BR EACCES .
+This difference originates from the design of locks in the SMB
+protocol, which provides mandatory locking semantics. The
+.I nobrl
+mount option (see
+.BR mount.cifs (8))
+turns off
+.BR fnctl (2)
+and
+.BR flock ()
+lock propagation to remote clients and makes
+.BR flock ()
+locks advisory again.
 .SH SEE ALSO
 .BR flock (1),
 .BR close (2),
-- 
2.30.0