Re: [PATCH v2] proc/mounts: add cursor

[Al Viro <viro@zeniv.linux.org.uk>]

On Thu, Apr 09, 2020 at 04:16:19PM +0200, Miklos Szeredi wrote:
> Solve this by adding a cursor entry for each open instance.  Taking the
> global namespace_sem for write seems excessive, since we are only dealing
> with a per-namespace list.  Instead add a per-namespace spinlock and use
> that together with namespace_sem taken for read to protect against
> concurrent modification of the mount list.  This may reduce parallelism of
> is_local_mountpoint(), but it's hardly a big contention point.  We could
> also use RCU freeing of cursors to make traversal not need additional
> locks, if that turns out to be neceesary.

Umm...  That can do more than reduction of parallelism - longer lists take
longer to scan and moving cursors dirties cachelines in a bunch of struct
mount instances.  And I'm not convinced that your locking in m_next() is
correct.

What's to stop umount_tree() from removing the next entry from the list
just as your m_next() tries to move the cursor?  I don't see any common
locks for those two...