userfaultfd: protect mmap_changing with rw_sem in userfaulfd_ctx

Increments and loads to mmap_changing are always in mmap_lock critical
section.  This ensures that if userspace requests event notification for
non-cooperative operations (e.g.  mremap), userfaultfd operations don't
occur concurrently.

This can be achieved by using a separate read-write semaphore in
userfaultfd_ctx such that increments are done in write-mode and loads in
read-mode, thereby eliminating the dependency on mmap_lock for this
purpose.

This is a preparatory step before we replace mmap_lock usage with per-vma
locks in fill/move ioctls.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Lokesh Gidra <[email protected]>
Reviewed-by: Mike Rapoport (IBM) <[email protected]>
Reviewed-by: Liam R. Howlett <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Axel Rasmussen <[email protected]>
Cc: Brian Geffon <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Kalesh Singh <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Nicolas Geoffray <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Tim Murray <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

0 files changed, 0 insertions, 0 deletions