[PATCH] rwsem: Update comments on rwsem count for count < WAITING_BIAS

[Tim Chen]

Update the comments for rwsem count for the case
where count < WAITING_BIAS.  Also some clean up of comments
and added explanation on how the rwsem_down_read_failed
path uses the count field.

Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
---
 kernel/locking/rwsem-xadd.c | 39 +++++++++++++++++++++++++--------------
 1 file changed, 25 insertions(+), 14 deletions(-)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index b4219ff..a794aaa 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -12,24 +12,24 @@
 #include <linux/export.h>
 
 /*
- * Guide to the rw_semaphore's count field for common values.
- * (32-bit case illustrated, similar for 64-bit)
+ * Guide to the rw_semaphore's count field.
+ * (32-bit count illustrated in descending order, similar for 64-bit count)
  *
  * 0x0000000X	(1) X readers active or attempting lock, no writer waiting
- *		    X = #active_readers + #readers attempting to lock
- *		    (X*ACTIVE_BIAS)
+ *		    where X = #active_readers + #readers attempting to lock
+ *		    count computed as (X*ACTIVE_BIAS)
  *
- * 0x00000000	rwsem is unlocked, and no one is waiting for the lock or
- *		attempting to read lock or write lock.
+ * 0x00000000	(1) 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
+ *		    where 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
+ *		    where 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
+ *		    where 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
@@ -43,19 +43,30 @@
  *		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)
+ * count < WAITING_BIAS
+ *		(1) X writer active, Y writer(s) attempting lock,
+ *		    Z readers attempting lock, no waiters
+ *		    where X = 0 or 1, (X+Y) >= 2, Z >= 0
+ *                  ((X+Y) * ACTIVE_WRITE_BIAS + Z * ACTIVE_BIAS)
+ *		(2) X writer active, Y writer(s) attempting lock,
+ *		    Z readers attempting lock, waiters for lock 
+ *		    where X = 0 or 1, (X+Y) >= 1, Z >= 0
+ *		    ((X+Y) * ACTIVE_WRITE_BIAS + Z * ACTIVE_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).
+ *	 i.e. the case where there are only readers locking or nobody has lock.
+ *	 (1st and 2nd case above). In rwsem_down_read failed, after 
+ *	 putting itself on the wait queue, it will check again if there are
+ *	 only readers locking, nobody has lock or it is first in queue (1, 2, and
+ *	 5th case), and call __rwsem_do_wake to wake up waiter at front 
+ *	 of queue to attempt locking again.
  *
  *	 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
+ *	 are only waiters but none active (5th case), and attempt to
  *	 steal the lock.
  *
  */
-- 
1.7.11.7




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