On Wed, 29 Jun 2022, Dave Chinner wrote: > On Tue, Jun 28, 2022 at 05:19:12PM -0600, James Yonan wrote: > > On 6/28/22 12:34, Amir Goldstein wrote: > > > On Tue, Jun 28, 2022 at 8:44 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: > > > > On Mon, Jun 27, 2022 at 04:11:07PM -0600, James Yonan wrote: > > > > > > > > > && d_is_positive(new_dentry) > > > > > && timespec64_compare(&d_backing_inode(old_dentry)->i_mtime, > > > > > &d_backing_inode(new_dentry)->i_mtime) <= 0) > > > > > goto exit5; > > > > > > > > > > It's pretty cool in a way that a new atomic file operation can even be > > > > > implemented in just 5 lines of code, and it's thanks to the existing > > > > > locking infrastructure around file rename/move that these operations > > > > > become almost trivial. Unfortunately, every fs must approve a new > > > > > renameat2() flag, so it bloats the patch a bit. > > > > How is it atomic and what's to stabilize ->i_mtime in that test? > > > > Confused... > > > Good point. > > > RENAME_EXCHANGE_WITH_NEWER would have been better > > > in that regard. > > > > > > And you'd have to check in vfs_rename() after lock_two_nondirectories() > > > > So I mean atomic in the sense that you are comparing the old and new mtimes > > inside the lock_rename/unlock_rename critical section in do_renameat2(), so > > mtime is not stable during rename, even with the inode locked. e.g. a > write page fault occurring concurrently with rename will change > mtime, and so which inode is "newer" can change during the rename > syscall... I don't think that is really important for the proposed use case. In any case where you might be using this new rename flag, the target file wouldn't be open for write, so the mtime wouldn't change. The atomicity is really wanted to make sure the file at the destination name is still the one that was expected (I think). So I think it would be reasonable for the rename to fail if the target file (or even "either file") is open for write. Can that change while the inode is locked? It would be nice if renameat2 took a third fd so we could say "only rename if <this> is the target file", but it doesn't. NeilBrown
