[RFC PATCH 09/11] mm/hmm: Remove racy protection against double-unregistration

From: Jason Gunthorpe <jgg@ziepe.ca>
To: linux-rdma@vger.kernel.org, linux-mm@kvack.org,
	Jerome Glisse <jglisse@redhat.com>,
	Ralph Campbell <rcampbell@nvidia.com>,
	John Hubbard <jhubbard@nvidia.com>
Cc: Jason Gunthorpe <jgg@mellanox.com>
Subject: [RFC PATCH 09/11] mm/hmm: Remove racy protection against double-unregistration
Date: Thu, 23 May 2019 12:34:34 -0300	[thread overview]
Message-ID: <20190523153436.19102-10-jgg@ziepe.ca> (raw)
In-Reply-To: <20190523153436.19102-1-jgg@ziepe.ca>

From: Jason Gunthorpe <jgg@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@mellanox.com>
---
 mm/hmm.c | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index 46872306f922bb..6c3b7398672c29 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -286,18 +286,13 @@ 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

