[PATCH v2 0/2] ipc,namespace: fix free vs allocation race

[Rik van Riel]

The IPC namespace code frees ipc_namespace structures asynchronously,
via a work queue item. This results in ipc_namespace structures being
freed very slowly, and the allocation path getting false failures
since the to-be-freed ipc_namespace structures have not been freed
yet.

Fix that by having the allocator wait when there are ipc_namespace
structures pending to be freed.

Also speed up the freeing of ipc_namespace structures. We had some
discussions about this last year, and ended up trying out various
"nicer" ideas that did not work, so I went back to the original,
with Al Viro's suggestion for a helper function:

https://lore.kernel.org/all/Yg8StKzTWh+7FLuA@xxxxxxxxxxxxxxxxxxxxx/

This series fixes both the false allocation failures, and the slow
freeing of ipc_namespace structures.

v2: a few more fs/namespace.c cleanups suggested by Al Viro (thank you!)