Re: 2.6.24-rc7 lockdep warning when poweroff

[Johannes Berg <johannes@sipsolutions.net>]

Ok, I checked all users of the create*workqueue() API now.

Turns out that there are many users that give a dynamic string as the
workqueue name (only the first three are relevant for the problem at
hand because the others are single-threaded):

drivers/connector/cn_queue.c
drivers/media/video/ivtv/ivtv-driver.c
drivers/message/i2o/driver.c

drivers/i2c/chips/m41t00.c drivers/infiniband/core/mad.c
drivers/message/fusion/mptfc.c drivers/net/qla3xxx.c
drivers/scsi/hosts.c drivers/scsi/qla4xxx/ql4_os.c
drivers/scsi/scsi_transport_fc.c drivers/spi/mpc52xx_psc_spi.c
drivers/spi/omap2_mcspi.c drivers/spi/spi_bitbang.c
drivers/spi/spi_txx9.c drivers/spi/spi_imx.c drivers/spi/pxa2xx_spi.c
drivers/spi/spi_bfin5xx.c drivers/power/ds2760_battery.c
net/mac80211/ieee80211.c


That's not really the problem though.

TBH, when writing the workqueue deadlock detection code I wasn't aware
that it is not allowed to use the same key with different names.

To make sure now:
	same key - different name	- BAD
	same key - same name		- OK
	different key - same name	- OK
	different key - different name	- OK

Correct?

The root problem here seems to be that I use the same name as for the
workqueue for the lockdep_map and other code uses a non-static workqueue
name. Using the workqueue name for the lock is good for knowing which
workqueue ran into trouble though.

mac80211 for example wants to allocate a (single-threaded) workqueue for
each hardware that is plugged in and wants to call it by the hardware
name.

Anyway, the patch below should help. I hope the patch compiles, I don't
have a lockdep-enabled system at hand right now (irqtrace is still not
supported on powerpc and my 64-bit powerpc isn't running a kernel with
my irqtrace support patch at the moment).

If you think the patch is a correct way to solve the problem I'll submit
it formally and it should then be included in 2.6.24 to avoid
regressions with the workqueue API (the workqueue lockup detection was
merged early in 2.6.24.) Who should I send it to in that case?

Dave, do you know if you had connector, ivtv or i2o in the kernel (just
to make sure my analysis was correct)? And can you reproduce the
problem, and if so, can you try if this patch helps?

johannes


---
 include/linux/workqueue.h |   14 +++++++++++---
 kernel/workqueue.c        |    5 +++--
 2 files changed, 14 insertions(+), 5 deletions(-)

--- everything.orig/include/linux/workqueue.h	2008-01-15 02:10:55.098131131 +0100
+++ everything/include/linux/workqueue.h	2008-01-15 02:26:37.428130426 +0100
@@ -149,19 +149,27 @@ struct execute_work {
 
 extern struct workqueue_struct *
 __create_workqueue_key(const char *name, int singlethread,
-		       int freezeable, struct lock_class_key *key);
+		       int freezeable, struct lock_class_key *key,
+		       const char *lock_name);
 
 #ifdef CONFIG_LOCKDEP
 #define __create_workqueue(name, singlethread, freezeable)	\
 ({								\
 	static struct lock_class_key __key;			\
+	const char *__lock_name;				\
+								\
+	if (__builtin_constant_p(name))				\
+		__lock_name = (name);				\
+	else							\
+		__lock_name = #name;				\
 								\
 	__create_workqueue_key((name), (singlethread),		\
-			       (freezeable), &__key);		\
+			       (freezeable), &__key,		\
+			       __lock_name);			\
 })
 #else
 #define __create_workqueue(name, singlethread, freezeable)	\
-	__create_workqueue_key((name), (singlethread), (freezeable), NULL)
+	__create_workqueue_key((name), (singlethread), (freezeable), NULL, NULL)
 #endif
 
 #define create_workqueue(name) __create_workqueue((name), 0, 0)
--- everything.orig/kernel/workqueue.c	2008-01-15 02:15:13.578132867 +0100
+++ everything/kernel/workqueue.c	2008-01-15 02:18:40.518138455 +0100
@@ -722,7 +722,8 @@ static void start_workqueue_thread(struc
 struct workqueue_struct *__create_workqueue_key(const char *name,
 						int singlethread,
 						int freezeable,
-						struct lock_class_key *key)
+						struct lock_class_key *key,
+						const char *lock_name)
 {
 	struct workqueue_struct *wq;
 	struct cpu_workqueue_struct *cwq;
@@ -739,7 +740,7 @@ struct workqueue_struct *__create_workqu
 	}
 
 	wq->name = name;
-	lockdep_init_map(&wq->lockdep_map, name, key, 0);
+	lockdep_init_map(&wq->lockdep_map, lock_name, key, 0);
 	wq->singlethread = singlethread;
 	wq->freezeable = freezeable;
 	INIT_LIST_HEAD(&wq->list);