random: simplify accounting logic

This logic is exactly equivalent to the old logic, but it should
be easier to see what it's doing.

The equivalence depends on one fact from outside this function:
when 'r->limit' is false, 'reserved' is zero.  (Well, two facts;
the other is that 'reserved' is never negative.)

Cc: Jiri Kosina <jkosina@suse.cz>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Greg Price <price@mit.edu>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 4 insertions, 8 deletions

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 2c532a6b0a21..9675821b4b5a 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -984,14 +984,10 @@ retry:
                 ibytes = 0;
         } else {
                 /* If limited, never pull more than available */
-                if (r->limit && ibytes + reserved >= have_bytes)
-                        ibytes = have_bytes - reserved;
-
-                if (have_bytes >= ibytes + reserved)
-                        entropy_count -= ibytes << (ENTROPY_SHIFT + 3);
-                else
-                        entropy_count = reserved << (ENTROPY_SHIFT + 3);
-
+                if (r->limit)
+                        ibytes = min_t(size_t, ibytes, have_bytes - reserved);
+                entropy_count = max_t(int, 0,
+                            entropy_count - (ibytes << (ENTROPY_SHIFT + 3)));
                 if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig)
                         goto retry;