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...
