sh: lockless UTLB miss fast-path.

With the refactored update_mmu_cache() introduced in older kernels,
there's no longer any need to take the page_table_lock in this path,
so simply drop it completely.

Without this, performance degradation is seen on SMP on heavily
threaded workloads that don't use the split ptlock, and ultimately
we have no reason to contend for the lock in the first place.
Signed-off-by: Paul Mundt <lethal@linux-sh.org>

arch/sh/mm/fault.c

@@ -258,9 +258,6 @@ asmlinkage int __kprobes __do_page_fault(struct pt_regs *regs,
 	pmd_t *pmd;
 	pte_t *pte;
 	pte_t entry;
-	struct mm_struct *mm = current->mm;
-	spinlock_t *ptl = NULL;
-	int ret = 1;
 
 #ifdef CONFIG_SH_KGDB
 	if (kgdb_nofault && kgdb_bus_err_hook)
@@ -274,12 +271,11 @@ asmlinkage int __kprobes __do_page_fault(struct pt_regs *regs,
 	 */
 	if (address >= P3SEG && address < P3_ADDR_MAX) {
 		pgd = pgd_offset_k(address);
-		mm = NULL;
 	} else {
-		if (unlikely(address >= TASK_SIZE || !mm))
+		if (unlikely(address >= TASK_SIZE || !current->mm))
 			return 1;
 
-		pgd = pgd_offset(mm, address);
+		pgd = pgd_offset(current->mm, address);
 	}
 
 	pud = pud_offset(pgd, address);
@@ -289,16 +285,12 @@ asmlinkage int __kprobes __do_page_fault(struct pt_regs *regs,
 	if (pmd_none_or_clear_bad(pmd))
 		return 1;
 
-	if (mm)
-		pte = pte_offset_map_lock(mm, pmd, address, &ptl);
-	else
 	pte = pte_offset_kernel(pmd, address);
-
 	entry = *pte;
 	if (unlikely(pte_none(entry) || pte_not_present(entry)))
-		goto unlock;
+		return 1;
 	if (unlikely(writeaccess && !pte_write(entry)))
-		goto unlock;
+		return 1;
 
 	if (writeaccess)
 		entry = pte_mkdirty(entry);
@@ -306,9 +298,6 @@ asmlinkage int __kprobes __do_page_fault(struct pt_regs *regs,
 
 	set_pte(pte, entry);
 	update_mmu_cache(NULL, address, entry);
-	ret = 0;
-unlock:
-	if (mm)
-		pte_unmap_unlock(pte, ptl);
-	return ret;
+
+	return 0;
 }