On Wed, Sep 4, 2019 at 4:29 PM Al Viro viro@zeniv.linux.org.uk wrote:
On Wed, Sep 04, 2019 at 03:38:20PM -0700, Linus Torvalds wrote:
On Wed, Sep 4, 2019 at 3:31 PM David Howells dhowells@redhat.com wrote:
It ought to be reasonably easy to make them per-sb at least, I think. We don't allow cross-super rename, right?
Right now the sequence count handling very much depends on it being a global entity on the reader side, at least.
And while the rename sequence count could (and probably should) be per-sb, the same is very much not true of the mount one.
Huh? That will cost us having to have a per-superblock dentry hash table; recall that lockless lockup can give false negatives if something gets moved from chain to chain, and rename_lock is first and foremost used to catch those and retry. If we split it on per-superblock basis, we can't have dentries from different superblocks in the same chain anymore...
That's exactly the "very much depends on it being a global entity on the reader side" thing.
I'm not convinced that's the _only_ way to handle things. Maybe a combination of (wild handwaving) per-hashqueue sequence count and some clever scheme for pathname handling could work.
I've not personally seen a load where the global rename lock has been a problem (very few things really do a lot of renames), but system-wide locks do make me nervous.
We have other (and worse) ones. tasklist_lock comes to mind.
Linus