mmap locking API: add mmap_read_trylock_non_owner()

Add a couple APIs used by kernel/bpf/stackmap.c only:
- mmap_read_trylock_non_owner()
- mmap_read_unlock_non_owner() (may be called from a work queue).

It's still not ideal that bpf/stackmap subverts the lock ownership in this
way.  Thanks to Peter Zijlstra for suggesting this API as the least-ugly
way of addressing this in the short term.

Signed-off-by: Michel Lespinasse <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Daniel Jordan <[email protected]>
Reviewed-by: Vlastimil Babka <[email protected]>
Reviewed-by: Davidlohr Bueso <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Jerome Glisse <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Laurent Dufour <[email protected]>
Cc: Liam Howlett <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ying Han <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 14 insertions, 0 deletions

diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index a757cb30ae77..d1826ce42f00 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -56,4 +56,18 @@ static inline void mmap_read_unlock(struct mm_struct *mm)
 	up_read(&mm->mmap_sem);
 }
 
+static inline bool mmap_read_trylock_non_owner(struct mm_struct *mm)
+{
+	if (down_read_trylock(&mm->mmap_sem)) {
+		rwsem_release(&mm->mmap_sem.dep_map, _RET_IP_);
+		return true;
+	}
+	return false;
+}
+
+static inline void mmap_read_unlock_non_owner(struct mm_struct *mm)
+{
+	up_read_non_owner(&mm->mmap_sem);
+}
+
 #endif /* _LINUX_MMAP_LOCK_H */