linux-kernel - [RFC][PATCH 6/9] mm/gup: Fix the lockless walkers

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Date:   Mon, 30 Nov 2020 12:27:11 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org, willy@...radead.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        aarcange@...hat.com, kirill.shutemov@...ux.intel.com,
        jroedel@...e.de, peterz@...radead.org
Subject: [RFC][PATCH 6/9] mm/gup: Fix the lockless walkers

On architectures where the PTE/PMD is larger than the native word size
(i386-PAE for example), READ_ONCE() can do the wrong thing. Use
pmdp_get_lockless() just like we use ptep_get_lockless().

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/events/core.c |    2 +-
 mm/gup.c             |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7041,7 +7041,7 @@ static u64 perf_get_pgtable_size(struct
 		return pud_leaf_size(pud);
 
 	pmdp = pmd_offset_lockless(pudp, pud, addr);
-	pmd = READ_ONCE(*pmdp);
+	pmd = pmdp_get_lockless(pmdp);
 	if (!pmd_present(pmd))
 		return 0;
 
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2467,7 +2467,7 @@ static int gup_pmd_range(pud_t *pudp, pu
 
 	pmdp = pmd_offset_lockless(pudp, pud, addr);
 	do {
-		pmd_t pmd = READ_ONCE(*pmdp);
+		pmd_t pmd = pmdp_get_lockless(pmdp);
 
 		next = pmd_addr_end(addr, end);
 		if (!pmd_present(pmd))