From: Aurelien Aptel <aaptel@xxxxxxxx> 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 <aaptel@xxxxxxxx> --- 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 . +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
