arch/tile: fix up locking in pgtable.c slightly

We should be holding the init_mm.page_table_lock in shatter_huge_page()
since we are modifying the kernel page tables.  Then, only if we are
walking the other root page tables to update them, do we want to take
the pgd_lock.

Add a comment about taking the pgd_lock that we always do it with
interrupts disabled and therefore are not at risk from the tlbflush
IPI deadlock as is seen on x86.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>

diff --git a/arch/tile/mm/pgtable.c b/arch/tile/mm/pgtable.c
index 6b9a109..2410aa8 100644 (file)
--- a/arch/tile/mm/pgtable.c
+++ b/arch/tile/mm/pgtable.c
@@ -177,14 +177,10 @@ void shatter_huge_page(unsigned long addr)
        if (!pmd_huge_page(*pmd))
                return;
 
-       /*
-        * Grab the pgd_lock, since we may need it to walk the pgd_list,
-        * and since we need some kind of lock here to avoid races.
-        */
-       spin_lock_irqsave(&pgd_lock, flags);
+       spin_lock_irqsave(&init_mm.page_table_lock, flags);
        if (!pmd_huge_page(*pmd)) {
                /* Lost the race to convert the huge page. */
-               spin_unlock_irqrestore(&pgd_lock, flags);
+               spin_unlock_irqrestore(&init_mm.page_table_lock, flags);
                return;
        }
 
@@ -194,6 +190,7 @@ void shatter_huge_page(unsigned long addr)
 
 #ifdef __PAGETABLE_PMD_FOLDED
        /* Walk every pgd on the system and update the pmd there. */
+       spin_lock(&pgd_lock);
        list_for_each(pos, &pgd_list) {
                pmd_t *copy_pmd;
                pgd = list_to_pgd(pos) + pgd_index(addr);
@@ -201,6 +198,7 @@ void shatter_huge_page(unsigned long addr)
                copy_pmd = pmd_offset(pud, addr);
                __set_pmd(copy_pmd, *pmd);
        }
+       spin_unlock(&pgd_lock);
 #endif
 
        /* Tell every cpu to notice the change. */
@@ -208,7 +206,7 @@ void shatter_huge_page(unsigned long addr)
                     cpu_possible_mask, NULL, 0);
 
        /* Hold the lock until the TLB flush is finished to avoid races. */
-       spin_unlock_irqrestore(&pgd_lock, flags);
+       spin_unlock_irqrestore(&init_mm.page_table_lock, flags);
 }
 
 /*
@@ -217,9 +215,13 @@ void shatter_huge_page(unsigned long addr)
  * against pageattr.c; it is the unique case in which a valid change
  * of kernel pagetables can't be lazily synchronized by vmalloc faults.
  * vmalloc faults work because attached pagetables are never freed.
- * The locking scheme was chosen on the basis of manfred's
- * recommendations and having no core impact whatsoever.
- * -- wli
+ *
+ * The lock is always taken with interrupts disabled, unlike on x86
+ * and other platforms, because we need to take the lock in
+ * shatter_huge_page(), which may be called from an interrupt context.
+ * We are not at risk from the tlbflush IPI deadlock that was seen on
+ * x86, since we use the flush_remote() API to have the hypervisor do
+ * the TLB flushes regardless of irq disabling.
  */
 DEFINE_SPINLOCK(pgd_lock);
 LIST_HEAD(pgd_list);