Re: [PATCH] namei: implemented RENAME_NEWER flag for renameat2() conditional replace

[James Yonan <james@openvpn.net>]

On 6/28/22 20:35, Dave Chinner wrote:
> How do you explain it the API
> semantics to an app developer that might want to use this
> functionality? RENAME_EXCHANGE_WITH_NEWER would be atomic in the
> sense you either get the old or new file at the destination, but
> it's not atomic in the sense that it is serialised against all other
> potential modification operations against either the source or
> destination. Hence the "if newer" comparison is not part of the
> "atomic rename" operation that is supposedly being performed...

So the current proposal based on feedback is to move the mtime 
comparison to vfs_rename() to take advantage of existing 
{lock,unlock}_two_nondirectories critical section, then nest another 
critical section {deny,allow}_write_access (adapted to inodes) to 
stabilize the mtime.  The proposed use case never needs to compare 
mtimes of files that are open for write, and the plan would be to return 
-ETXTBSY in this case.

> I'm also sceptical of the use of mtime - we can't rely on mtime to
> determine the newer file accurately on all filesystems. e.g. Some
> fileystems only have second granularity in their timestamps, so
> there's a big window where "newer" cannot actually be determined by
> timestamp comparisons.
So in the "use a directory as a key/value store" use case in distributed 
systems, the file mtime is generally determined remotely by the file 
content creator and is set locally via futimens() rather than the local 
system clock.  So this gives you nanosecond scale time resolution if the 
content creator supports it, even if the system clock has less resolution.

James