powerpc: Prevent gcc to re-read the pagetables

GCC is very likely to read the pagetables just once and cache them in
the local stack or in a register, but it is can also decide to re-read
the pagetables. The problem is that the pagetable in those places can
change from under gcc.

With THP/hugetlbfs the pmd (and pgd for hugetlbfs giga pages) can
change under gup_fast. The pages won't be freed untill we finish
gup fast because we have irq disabled and we free these pages via
rcu callback.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

2 files changed, 5 insertions, 5 deletions

diff --git a/arch/powerpc/mm/gup.c b/arch/powerpc/mm/gup.c
index 223a255545fc..49822d90ea96 100644
--- a/arch/powerpc/mm/gup.c
+++ b/arch/powerpc/mm/gup.c
@@ -34,7 +34,7 @@ static noinline int gup_pte_range(pmd_t pmd, unsigned long addr,
 
         ptep = pte_offset_kernel(&pmd, addr);
         do {
-                pte_t pte = *ptep;
+                pte_t pte = ACCESS_ONCE(*ptep);
                 struct page *page;
 
                 if ((pte_val(pte) & mask) != result)
@@ -63,7 +63,7 @@ static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
 
         pmdp = pmd_offset(&pud, addr);
         do {
-                pmd_t pmd = *pmdp;
+                pmd_t pmd = ACCESS_ONCE(*pmdp);
 
                 next = pmd_addr_end(addr, end);
                 /*
@@ -97,7 +97,7 @@ static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
 
         pudp = pud_offset(&pgd, addr);
         do {
-                pud_t pud = *pudp;
+                pud_t pud = ACCESS_ONCE(*pudp);
 
                 next = pud_addr_end(addr, end);
                 if (pud_none(pud))
@@ -160,7 +160,7 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write,
 
         pgdp = pgd_offset(mm, addr);
         do {
-                pgd_t pgd = *pgdp;
+                pgd_t pgd = ACCESS_ONCE(*pgdp);
 
                 pr_devel("  %016lx: normal pgd %p\n", addr,
                          (void *)pgd_val(pgd));
diff --git a/arch/powerpc/mm/hugetlbpage.c b/arch/powerpc/mm/hugetlbpage.c
index e9e6882231da..f2f01fd3ec68 100644
--- a/arch/powerpc/mm/hugetlbpage.c
+++ b/arch/powerpc/mm/hugetlbpage.c
@@ -1024,7 +1024,7 @@ int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
         if (pte_end < end)
                 end = pte_end;
 
-        pte = *ptep;
+        pte = ACCESS_ONCE(*ptep);
         mask = _PAGE_PRESENT | _PAGE_USER;
         if (write)
                 mask |= _PAGE_RW;