linux-kernel - [PATCH 14/22] mm/hmm: Remove racy protection against double-unregistration

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Message-Id: <20190701062020.19239-15-hch@lst.de>
Date:   Mon,  1 Jul 2019 08:20:12 +0200
From:   Christoph Hellwig <hch@....de>
To:     Dan Williams <dan.j.williams@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Ben Skeggs <bskeggs@...hat.com>
Cc:     Ira Weiny <ira.weiny@...el.com>, linux-mm@...ck.org,
        nouveau@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-nvdimm@...ts.01.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, John Hubbard <jhubbard@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Philip Yang <Philip.Yang@....com>
Subject: [PATCH 14/22] mm/hmm: Remove racy protection against double-unregistration

From: Jason Gunthorpe <jgg@...lanox.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@...lanox.com>
Reviewed-by: Jérôme Glisse <jglisse@...hat.com>
Reviewed-by: John Hubbard <jhubbard@...dia.com>
Reviewed-by: Ralph Campbell <rcampbell@...dia.com>
Reviewed-by: Christoph Hellwig <hch@....de>
Tested-by: Philip Yang <Philip.Yang@....com>
---
 mm/hmm.c | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index 6f5dc6d568fe..2ef14b2b5505 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);
-- 
2.20.1