linux-kernel - RE: [PATCH v1 0/7] Remove in-tree usage of MAP

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Date:   Fri, 20 Aug 2021 08:25:47 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'NeilBrown' <neilb@...e.de>,
        "J. Bruce Fields" <bfields@...ldses.org>
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Subject: RE: [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE

From: NeilBrown
> Sent: 20 August 2021 04:45
...
> O_DENYREAD is an insane flag.  If a process reads a file that some other
> process is working on, then the only which could be hurt is the reader.
> So allowing a process to ask for the open to fail if someone is writing
> might make sense.  Insisting that all opens fail does not.
> Any code wanting O_DENYREAD *should* use advisory locking, and any code
> wanting to know about read denial should too.

It might make sense if O_DENYREAD | O_DENYWRITE | O_RDWR are all set.
That would be what O_EXCL ought to mean for a normal file.
So would be useful for a program that wants to update a config file.

...
> It would be nice to be able to combine O_DENYWRITE with O_RDWR.  This
> combination is exactly what the kernel *should* do for swap files.

I suspect that is a common usage - eg for updating a file that contains
a log file sequence number.

...
> I'm not sure about O_DENYDELETE.  It is a lock on the name.  Unix has
> traditionally used lock-files to lock a name.  The functionality makes
> sense for processes with write-access to the directory...

I'm not sure it makes any sense on filesystems that use inode numbers.
Which name would you protect, and how would you manage to do the test.
On windows O_DENYDELETE is pretty much the default.
Which is why software updates are such a PITA.

	David

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