x86: some lock annotations for user copy paths

copy_to/from_user and all its variants (except the atomic ones) can take a
page fault and perform non-trivial work like taking mmap_sem and entering
the filesyste/pagecache.

Unfortunately, this often escapes lockdep because a common pattern is to
use it to read in some arguments just set up from userspace, or write data
back to a hot buffer. In those cases, it will be unlikely for page reclaim
to get a window in to cause copy_*_user to fault.

With the new might_lock primitives, add some annotations to x86. I don't
know if I caught all possible faulting points (it's a bit of a maze, and I
didn't really look at 32-bit). But this is a starting point.

Boots and runs OK so far.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/lib/usercopy_64.c

@@ -16,6 +16,8 @@
 do {									   \
 	long __d0, __d1, __d2;						   \
 	might_sleep();							   \
+	if (current->mm)						   \
+		might_lock_read(&current->mm->mmap_sem);		   \
 	__asm__ __volatile__(						   \
 		"	testq %1,%1\n"					   \
 		"	jz 2f\n"					   \
@@ -65,6 +67,8 @@ unsigned long __clear_user(void __user *addr, unsigned long size)
 {
 	long __d0;
 	might_sleep();
+	if (current->mm)
+		might_lock_read(&current->mm->mmap_sem);
 	/* no memory constraint because it doesn't change any memory gcc knows
 	   about */
 	asm volatile(