MIPS: Make set_pte() SMP safe.
authorDavid Daney <david.daney@cavium.com>
Tue, 4 Aug 2015 00:48:43 +0000 (17:48 -0700)
committerRalf Baechle <ralf@linux-mips.org>
Wed, 5 Aug 2015 09:11:10 +0000 (11:11 +0200)
On MIPS the GLOBAL bit of the PTE must have the same value in any
aligned pair of PTEs.  These pairs of PTEs are referred to as
"buddies".  In a SMP system is is possible for two CPUs to be calling
set_pte() on adjacent PTEs at the same time.  There is a race between
setting the PTE and a different CPU setting the GLOBAL bit in its
buddy PTE.

This race can be observed when multiple CPUs are executing
vmap()/vfree() at the same time.

Make setting the buddy PTE's GLOBAL bit an atomic operation to close
the race condition.

The case of CONFIG_64BIT_PHYS_ADDR && CONFIG_CPU_MIPS32 is *not*
handled.

Signed-off-by: David Daney <david.daney@cavium.com>
Cc: <stable@vger.kernel.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/10835/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
arch/mips/include/asm/pgtable.h

index 9d81067..ae85694 100644 (file)
@@ -182,8 +182,39 @@ static inline void set_pte(pte_t *ptep, pte_t pteval)
                 * Make sure the buddy is global too (if it's !none,
                 * it better already be global)
                 */
+#ifdef CONFIG_SMP
+               /*
+                * For SMP, multiple CPUs can race, so we need to do
+                * this atomically.
+                */
+#ifdef CONFIG_64BIT
+#define LL_INSN "lld"
+#define SC_INSN "scd"
+#else /* CONFIG_32BIT */
+#define LL_INSN "ll"
+#define SC_INSN "sc"
+#endif
+               unsigned long page_global = _PAGE_GLOBAL;
+               unsigned long tmp;
+
+               __asm__ __volatile__ (
+                       "       .set    push\n"
+                       "       .set    noreorder\n"
+                       "1:     " LL_INSN "     %[tmp], %[buddy]\n"
+                       "       bnez    %[tmp], 2f\n"
+                       "        or     %[tmp], %[tmp], %[global]\n"
+                       "       " SC_INSN "     %[tmp], %[buddy]\n"
+                       "       beqz    %[tmp], 1b\n"
+                       "        nop\n"
+                       "2:\n"
+                       "       .set pop"
+                       : [buddy] "+m" (buddy->pte),
+                         [tmp] "=&r" (tmp)
+                       : [global] "r" (page_global));
+#else /* !CONFIG_SMP */
                if (pte_none(*buddy))
                        pte_val(*buddy) = pte_val(*buddy) | _PAGE_GLOBAL;
+#endif /* CONFIG_SMP */
        }
 #endif
 }