[PATCH] FUTEX_WAKE_OP: enhanced error handling

The code for FUTEX_WAKE_OP calls an arch callback,
futex_atomic_op_inuser(). That callback can return an error code, but
currently the caller assumes any error is EFAULT, and will try various
things to resolve the fault before eventually giving up with EFAULT
(regardless of the original error code). This is not a theoretical case -
arch callbacks currently return -ENOSYS if the opcode they are given is
bogus.

This patch alters the code to detect non-EFAULT errors and return them
directly to the user.

Of course, whether -ENOSYS is the correct return value for the bogus opcode
case, or whether EINVAL would be more appropriate is another question.

Signed-off-by: David Gibson <[e-mail]>
Cc: Rusty Russell <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jamie Lokier <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/futex.c

···
 		if (bh1 != bh2)
 			spin_unlock(&bh2->lock);
 
+		if (unlikely(op_ret != -EFAULT)) {
+			ret = op_ret;
+			goto out;
+		}
+
 		/* futex_atomic_op_inuser needs to both read and write
 		 * *(int __user *)uaddr2, but we can't modify it
 		 * non-atomically.  Therefore, if get_user below is not