1 files changed, 49 insertions, 0 deletions

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 1d66e08e897d..b4219ff87b8c 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -12,6 +12,55 @@
 #include <linux/export.h>
 
 /*
+ * Guide to the rw_semaphore's count field for common values.
+ * (32-bit case illustrated, similar for 64-bit)
+ *
+ * 0x0000000X   (1) X readers active or attempting lock, no writer waiting
+ *                  X = #active_readers + #readers attempting to lock
+ *                  (X*ACTIVE_BIAS)
+ *
+ * 0x00000000   rwsem is unlocked, and no one is waiting for the lock or
+ *              attempting to read lock or write lock.
+ *
+ * 0xffff000X   (1) X readers active or attempting lock, with waiters for lock
+ *                  X = #active readers + # readers attempting lock
+ *                  (X*ACTIVE_BIAS + WAITING_BIAS)
+ *              (2) 1 writer attempting lock, no waiters for lock
+ *                  X-1 = #active readers + #readers attempting lock
+ *                  ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
+ *              (3) 1 writer active, no waiters for lock
+ *                  X-1 = #active readers + #readers attempting lock
+ *                  ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
+ *
+ * 0xffff0001   (1) 1 reader active or attempting lock, waiters for lock
+ *                  (WAITING_BIAS + ACTIVE_BIAS)
+ *              (2) 1 writer active or attempting lock, no waiters for lock
+ *                  (ACTIVE_WRITE_BIAS)
+ *
+ * 0xffff0000   (1) There are writers or readers queued but none active
+ *                  or in the process of attempting lock.
+ *                  (WAITING_BIAS)
+ *              Note: writer can attempt to steal lock for this count by adding
+ *              ACTIVE_WRITE_BIAS in cmpxchg and checking the old count
+ *
+ * 0xfffe0001   (1) 1 writer active, or attempting lock. Waiters on queue.
+ *                  (ACTIVE_WRITE_BIAS + WAITING_BIAS)
+ *
+ * Note: Readers attempt to lock by adding ACTIVE_BIAS in down_read and checking
+ *       the count becomes more than 0 for successful lock acquisition,
+ *       i.e. the case where there are only readers or nobody has lock.
+ *       (1st and 2nd case above).
+ *
+ *       Writers attempt to lock by adding ACTIVE_WRITE_BIAS in down_write and
+ *       checking the count becomes ACTIVE_WRITE_BIAS for successful lock
+ *       acquisition (i.e. nobody else has lock or attempts lock).  If
+ *       unsuccessful, in rwsem_down_write_failed, we'll check to see if there
+ *       are only waiters but none active (5th case above), and attempt to
+ *       steal the lock.
+ *
+ */
+
+/*
  * Initialize an rwsem:
  */
 void __init_rwsem(struct rw_semaphore *sem, const char *name,