arc: call find_vma with the mmap_sem held

Performing vma lookups without taking the mm->mmap_sem is asking for
trouble.  While doing the search, the vma in question can be modified or
even removed before returning to the caller.  Take the lock (shared) in
order to avoid races while iterating through the vmacache and/or rbtree.

[akpm@linux-foundation.org: CSE current->active_mm, per Vineet]
Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Acked-by: Vineet Gupta <vgupta@synopsys.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/arc/kernel/troubleshoot.c

@@ -86,12 +86,13 @@ static void show_faulting_vma(unsigned long address, char *buf)
 	unsigned long ino = 0;
 	dev_t dev = 0;
 	char *nm = buf;
+	struct mm_struct *active_mm = current->active_mm;
 
 	/* can't use print_vma_addr() yet as it doesn't check for
 	 * non-inclusive vma
 	 */
-
-	vma = find_vma(current->active_mm, address);
+	down_read(&active_mm->mmap_sem);
+	vma = find_vma(active_mm, address);
 
 	/* check against the find_vma( ) behaviour which returns the next VMA
 	 * if the container VMA is not found
@@ -110,9 +111,10 @@ static void show_faulting_vma(unsigned long address, char *buf)
 			vma->vm_start < TASK_UNMAPPED_BASE ?
 				address : address - vma->vm_start,
 			nm, vma->vm_start, vma->vm_end);
-	} else {
+	} else
 		pr_info("    @No matching VMA found\n");
-	}
+
+	up_read(&active_mm->mmap_sem);
 }
 
 static void show_ecr_verbose(struct pt_regs *regs)