[PATCH v3 hmm 08/12] mm/hmm: Remove racy protection against double-unregistration

[Jason Gunthorpe]

From: Jason Gunthorpe <jgg at mellanox.com>

No other register/unregister kernel API attempts to provide this kind of
protection as it is inherently racy, so just drop it.

Callers should provide their own protection, it appears nouveau already
does, but just in case drop a debugging POISON.

Signed-off-by: Jason Gunthorpe <jgg at mellanox.com>
Reviewed-by: Jérôme Glisse <jglisse at redhat.com>
Reviewed-by: John Hubbard <jhubbard at nvidia.com>
Reviewed-by: Ralph Campbell <rcampbell at nvidia.com>
Tested-by: Philip Yang <Philip.Yang at amd.com>
---
 mm/hmm.c | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index c0f622f86223c2..e3e0a811a3a774 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -283,18 +283,13 @@ EXPORT_SYMBOL(hmm_mirror_register);
  */
 void hmm_mirror_unregister(struct hmm_mirror *mirror)
 {
-	struct hmm *hmm = READ_ONCE(mirror->hmm);
-
-	if (hmm == NULL)
-		return;
+	struct hmm *hmm = mirror->hmm;
 
 	down_write(&hmm->mirrors_sem);
 	list_del_init(&mirror->list);
-	/* To protect us against double unregister ... */
-	mirror->hmm = NULL;
 	up_write(&hmm->mirrors_sem);
-
 	hmm_put(hmm);
+	memset(&mirror->hmm, POISON_INUSE, sizeof(mirror->hmm));
 }
 EXPORT_SYMBOL(hmm_mirror_unregister);
 
-- 
2.21.0