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

From: Tim Chen
Date: Mon May 05 2014 - 06:53:08 EST


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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