linux-kernel - [PATCH RFC tip/core/rcu 13/15] mm/ksm: Remove now-redundant smp_read_barrier

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Date:   Mon,  9 Oct 2017 17:22:47 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, torvalds@...ux-foundation.org,
        mark.rutland@....com, dhowells@...hat.com,
        linux-arch@...r.kernel.org, peterz@...radead.org,
        will.deacon@....com,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Minchan Kim <minchan@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.vnet.ibm.com>,
        <linux-mm@...ck.org>
Subject: [PATCH RFC tip/core/rcu 13/15] mm/ksm: Remove now-redundant smp_read_barrier_depends()

Because READ_ONCE() now implies smp_read_barrier_depends(), the
smp_read_barrier_depends() in get_ksm_page() is now redundant.
This commit removes it and updates the comments.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Minchan Kim <minchan@...nel.org>
Cc: Michal Hocko <mhocko@...e.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc: Claudio Imbrenda <imbrenda@...ux.vnet.ibm.com>
Cc: <linux-mm@...ck.org>
---
 mm/ksm.c | 9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 6cb60f46cce5..7d70f923bc45 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -675,15 +675,8 @@ static struct page *get_ksm_page(struct stable_node *stable_node, bool lock_it)
 	expected_mapping = (void *)((unsigned long)stable_node |
 					PAGE_MAPPING_KSM);
 again:
-	kpfn = READ_ONCE(stable_node->kpfn);
+	kpfn = READ_ONCE(stable_node->kpfn); /* Address dependency. */
 	page = pfn_to_page(kpfn);
-
-	/*
-	 * page is computed from kpfn, so on most architectures reading
-	 * page->mapping is naturally ordered after reading node->kpfn,
-	 * but on Alpha we need to be more careful.
-	 */
-	smp_read_barrier_depends();
 	if (READ_ONCE(page->mapping) != expected_mapping)
 		goto stale;
 
-- 
2.5.2