Re: [PATCH v3] flock.2: add CIFS details

From: Aurelien Aptel <redacted>

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 EACCESS.  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 <redacted>
---
  man2/flock.2 | 29 +++++++++++++++++++++++++++++
  1 file changed, 29 insertions(+)

diff --git a/man2/flock.2 b/man2/flock.2
index 61d4b5396..4b6e5cc24 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 EACCESS .