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

["Aurélien Aptel" <aaptel@suse.com>]

Hi Tom,

Thanks for taking a look.

Tom Talpey <tom@talpey.com> writes:
> On 3/2/2021 10:48 AM, Aurélien Aptel wrote:
> I'd suggest removing this sentence. It doesn't really add anything to
> the definition.

OK.

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

This is about clients. Let's say the same app is running on 2 different
Linux system that have the same Windows Server share mounted.

The scenario is those 2 app instances use the same file on the share and
are trying to synchronize access using flock().

Pre-5.5, CIFS flock() is using the generic flock() implementation from
the Linux VFS layer which only knows about syscall made by local apps
and isn't aware that the file can be accessed under its feet from the
network.

In 5.5 and above, CIFS flock() is implemented using SMB locks, which
have different semantics than what POSIX defines, i.e. you cannot ignore
the locks and write, write() will fail with EPERM. So this version can
be used for file sync with other clients but works slightly
differently. It is a best-effort attempt.

Does this clarification changes anything to your suggestions?

> "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."


> "protocol, which provides mandatory locking semantics."

OK. As it turns out, there is actually a 'nobrl' mount option to get back
pre-5.5 behavior. I'll mention it and use your suggestions in v2.

Cheers,
-- 
Aurélien Aptel / SUSE Labs Samba Team
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