[PATCH v4 hmm 09/12] 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, and it appears nouveau
already does.

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>
Reviewed-by: Christoph Hellwig <hch at lst.de>
Tested-by: Philip Yang <Philip.Yang at amd.com>
---
v3
- Drop poison, looks like there are no new patches that will use this
  wrong (Christoph)
---
 mm/hmm.c | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index 6f5dc6d568feb1..2ef14b2b5505f6 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -276,17 +276,11 @@ 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);
 }
 EXPORT_SYMBOL(hmm_mirror_unregister);
-- 
2.22.0