xen: allow set_pte_at on init_mm to be lockless

The usual pagetable locking protocol doesn't seem to apply to updates
to init_mm, so don't rely on preemption being disabled in xen_set_pte_at
on init_mm.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 10 insertions, 2 deletions

diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 272635aaef88..6cbcf65609ad 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -156,6 +156,10 @@ void set_pte_mfn(unsigned long vaddr, unsigned long mfn, pgprot_t flags)
 void xen_set_pte_at(struct mm_struct *mm, unsigned long addr,
                     pte_t *ptep, pte_t pteval)
 {
+        /* updates to init_mm may be done without lock */
+        if (mm == &init_mm)
+                preempt_disable();
+
         if (mm == current->mm || mm == &init_mm) {
                 if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_MMU) {
                         struct multicall_space mcs;
@@ -163,12 +167,16 @@ void xen_set_pte_at(struct mm_struct *mm, unsigned long addr,
 
                         MULTI_update_va_mapping(mcs.mc, addr, pteval, 0);
                         xen_mc_issue(PARAVIRT_LAZY_MMU);
-                        return;
+                        goto out;
                 } else
                         if (HYPERVISOR_update_va_mapping(addr, pteval, 0) == 0)
-                                return;
+                                goto out;
         }
         xen_set_pte(ptep, pteval);
+
+out:
+        if (mm == &init_mm)
+                preempt_enable();
 }
 
 pteval_t xen_pte_val(pte_t pte)