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

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On 3/2/2021 10:48 AM, Aurélien Aptel wrote:
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.

I'd suggest removing this sentence. It doesn't really add anything to
the definition.

+.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.

This is discussing the scenario where a process on the server performs
an flock(), right? That's perhaps confusingly special. How about

"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."

+.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.

"protocol, which provides mandatory locking semantics."

  .SH SEE ALSO
  .BR flock (1),
  .BR close (2),




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