Re: [RFC PATCH] fs: introduce mkdirat2 syscall for atomic mkdir

On Mon, Mar 01, 2021 at 02:09:03PM -0500, J. Bruce Fields wrote:

On Sun, Feb 28, 2021 at 08:57:20AM -0500, Drew DeVault wrote:

quoted

On Sat Feb 27, 2021 at 11:03 PM EST, Matthew Wilcox wrote:

quoted

quoted

1. Program A creates a directory
2. Program A is pre-empted
3. Program B deletes the directory
4. Program A creates a file in that directory
5. RIP

umm ... program B deletes the directory. program A opens it in order to
use openat(). program A gets ENOENT and exits, confused. that's the
race you're removing here -- and it seems fairly insignificant to me.

Yes, that is the race being eliminated here. Instead of this, program A
has an fd which holds a reference to the directory, so it just works. A
race is a race. It's an oversight in the API.

Step 4 still fails either way, because you can't create a file in an
unlinked directory, even if you hold a reference to that directory.
What's the behavior change at step 4 that you're hoping for?

If step 3 is 'mv foo bar', then the behaviour change will be that the
files still get created, just as bar/quux, instead of foo/quux.  It's not
clear to me this is necessarily an improvement in behaviour.

(as an aside, i think there's a missing feature in posix -- being able
to atomically replace one directory with another.  you can atomically
replace one file with another with hard links, but since you can't
hardlink a directory, you can't do the same trick.  Maybe you should
just always move files out of a directory instead of moving directories
as a single operation)