random: simplify add_ptr logic

The add_ptr variable wasn't used in a sensible way, use only i instead.
i got reused later for a different purpose, use j instead.

While we're here, put tap0 first in the tap list and add a comment.
Signed-off-by: Matt Mackall <mpm@selenic.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/random.c

@@ -454,7 +454,7 @@ static void __add_entropy_words(struct entropy_store *r, const __u32 *in,
 	static __u32 const twist_table[8] = {
 		0x00000000, 0x3b6e20c8, 0x76dc4190, 0x4db26158,
 		0xedb88320, 0xd6d6a3e8, 0x9b64c2b0, 0xa00ae278 };
-	unsigned long i, add_ptr, tap1, tap2, tap3, tap4, tap5;
+	unsigned long i, j, tap1, tap2, tap3, tap4, tap5;
 	int input_rotate;
 	int wordmask = r->poolinfo->poolwords - 1;
 	__u32 w;
@@ -469,19 +469,21 @@ static void __add_entropy_words(struct entropy_store *r, const __u32 *in,
 
 	spin_lock_irqsave(&r->lock, flags);
 	input_rotate = r->input_rotate;
-	add_ptr = r->add_ptr;
+	i = r->add_ptr;
 
 	while (nwords--) {
 		w = rol32(*in++, input_rotate & 31);
-		i = add_ptr = (add_ptr - 1) & wordmask;
+		i = (i - 1) & wordmask;
 
 		/* XOR in the various taps */
+		w ^= r->pool[i];
 		w ^= r->pool[(i + tap1) & wordmask];
 		w ^= r->pool[(i + tap2) & wordmask];
 		w ^= r->pool[(i + tap3) & wordmask];
 		w ^= r->pool[(i + tap4) & wordmask];
 		w ^= r->pool[(i + tap5) & wordmask];
-		w ^= r->pool[i];
+
+		/* Mix the result back in with a twist */
 		r->pool[i] = (w >> 3) ^ twist_table[w & 7];
 
 		/*
@@ -494,14 +496,11 @@ static void __add_entropy_words(struct entropy_store *r, const __u32 *in,
 	}
 
 	r->input_rotate = input_rotate;
-	r->add_ptr = add_ptr;
+	r->add_ptr = i;
 
-	if (out) {
-		for (i = 0; i < 16; i++) {
-			out[i] = r->pool[add_ptr];
-			add_ptr = (add_ptr - 1) & wordmask;
-		}
-	}
+	if (out)
+		for (j = 0; j < 16; j++)
+			out[j] = r->pool[(i - j) & wordmask];
 
 	spin_unlock_irqrestore(&r->lock, flags);
 }