On Thu, Apr 09, 2020 at 05:50:48PM +0100, Al Viro wrote: > 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... Ah, you still have namespace_sem taken (shared) by m_start(). Nevermind that one, then... Let me get through mnt_list users and see if I can catch anything.
