[PATCH] random driver preempt robustness

A certain codepath in the random driver relied on vt_ioctl() being under
the BKL and implicitly disabling preemption.  The code wasn't buggy
upstream but it's slighly unrobust so I think we want the fix upstream too,
independently of the remove-bkl patch.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/char/random.c

@@ -807,10 +807,11 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
 	long		delta, delta2, delta3;
 	int		entropy = 0;
 
+	preempt_disable();
 	/* if over the trickle threshold, use only 1 in 4096 samples */
 	if ( random_state->entropy_count > trickle_thresh &&
 	     (__get_cpu_var(trickle_count)++ & 0xfff))
-		return;
+		goto out;
 
 	/*
 	 * Use get_cycles() if implemented, otherwise fall back to
@@ -861,6 +862,8 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
 		entropy = int_ln_12bits(delta);
 	}
 	batch_entropy_store(num, time, entropy);
+out:
+	preempt_enable();
 }
 
 void add_keyboard_randomness(unsigned char scancode)