rwsem: Add comments to explain the meaning of the rwsem's count field

[cascardo/linux.git] / kernel / locking / rwsem-xadd.c

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 1d66e08..b4219ff 100644 (file)
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -11,6 +11,55 @@
 #include <linux/init.h>
 #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:
  */