Date:	Sat, 5 Aug 2006 12:54:11 +0200
From:	"Michal Piotrowski" <michal.k.k.piotrowski@...il.com>
To:	"Dave Jones" <davej@...hat.com>,
	"Linus Torvalds" <torvalds@...l.org>,
	"Michal Piotrowski" <michal.k.k.piotrowski@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: 2.6.18-rc3-g3b445eea BUG: warning at /usr/src/linux-git/kernel/cpu.c:51/unlock_cpu_hotplug()

On 05/08/06, Dave Jones <davej@...hat.com> wrote:
> On Fri, Aug 04, 2006 at 10:49:47PM -0400, Dave Jones wrote:
>
> This trace now makes a lot more sense to me.
>
>  > CPU1 called lock_cpu_hotplug() for app cpuspeed. recursive_depth=0
>  >  [<c0104edc>] show_trace_log_lvl+0x58/0x152
>  >  [<c01054c2>] show_trace+0xd/0x10
>  >  [<c01055db>] dump_stack+0x19/0x1b
>  >  [<c013e8c3>] lock_cpu_hotplug+0x39/0xbf
>  >  [<c029fbae>] store_scaling_governor+0x142/0x1a3
>  >  [<c029f1a5>] store+0x37/0x48
>  >  [<c01a6561>] sysfs_write_file+0xab/0xd1
>  >  [<c016f99f>] vfs_write+0xab/0x157
>  >  [<c016ffe4>] sys_write+0x3b/0x60
>  >  [<c0103db9>] sysenter_past_esp+0x56/0x8d
>  > cpuspeed acquired cpu_bitmask_lock
>  >
>  > CPU1 called lock_cpu_hotplug() for app cpuspeed. recursive_depth=0
>  >  [<c0104edc>] show_trace_log_lvl+0x58/0x152
>  >  [<c01054c2>] show_trace+0xd/0x10
>  >  [<c01055db>] dump_stack+0x19/0x1b
>  >  [<c013e8c3>] lock_cpu_hotplug+0x39/0xbf
>  >  [<c0132f3c>] __create_workqueue+0x52/0x122
>  >  [<f901234b>] cpufreq_governor_dbs+0x9f/0x2c3 [cpufreq_ondemand]
>  >  [<c029f7b6>] __cpufreq_governor+0x57/0xd8
>  >  [<c029f985>] __cpufreq_set_policy+0x14e/0x1bc
>  >  [<c029fbc5>] store_scaling_governor+0x159/0x1a3
>  >  [<c029f1a5>] store+0x37/0x48
>  >  [<c01a6561>] sysfs_write_file+0xab/0xd1
>  >  [<c016f99f>] vfs_write+0xab/0x157
>  >  [<c016ffe4>] sys_write+0x3b/0x60
>  >  [<c0103db9>] sysenter_past_esp+0x56/0x8d
>  > Lukewarm IQ detected in hotplug locking
>  > BUG: warning at kernel/cpu.c:46/lock_cpu_hotplug()
>
> So when we write to sysfs to set the governor, we end up in store_scaling_governor()
> which takes the hotplug lock, and then calls off into the governor to let it
> do its thing. Part of ondemand's "thing" is to create a workqueue.
> unfortunatly, __create_workqueue also takes the hotplug lock.
>
> Creating a variant of __create_workqueue that doesn't take the lock
> seems really nasty.
>
> We could remove the locking from store_scaling_governor() and make the governors
> themselves have to do the locking, but I'm not sure that's entirely safe.
>
> We could do something really disgusting like ...
>
>         unlock_cpu_hotplug()
>         ...
>         create_workqueue()
>         ...
>         lock_cpu_hotplug()
>
> in ondemand, which opens up a tiny race window, but as ugly as it is,
> looks to be the best solution of the bunch right now.
>
> Comments?
>
> The really sad part is this is completely unrelated to the original bug reported
> in this thread, which shows just how widespread this braindamage is.
> Michal's traces really don't really scream anything obvious to me.
> (Though given it took me 4 hours to decode my own traces above, this is no
> real sign of how big a problem this might be).
>
> Michal, could you apply this diff.. http://lkml.org/lkml/diff/2006/8/4/381/1
> (change the '120' to '60' first), and send me the debug spew that you get ?
> You'll have to wait until a minute of uptime has passed. Oh, and edit
> include/linux/jiffies.h to change INITIAL_JIFFIES to '0'.

p4-clockmod: P4/Xeon(TM) CPU On-Demand Clock Modulation available
ip_tables: (C) 2000-2006 Netfilter Core Team
Netfilter messages via NETLINK v0.30.
BUG: warning at /usr/src/linux-git/kernel/cpu.c:69/unlock_cpu_hotplug()
 [<c0132a91>] unlock_cpu_hotplug+0x63/0xb2
 [<c013a3a0>] stop_machine_run+0x2e/0x34
 [<c013573a>] sys_init_module+0x15a0/0x178a
 [<c015b86b>] do_sync_read+0xb6/0xf1
 [<c0102d51>] sysenter_past_esp+0x56/0x79
ip_conntrack version 2.4 (8192 buckets, 65536 max) - 224 bytes per conntrack
NET: Registered protocol family 17
skge eth0: enabling interface
skge eth0: Link is up at 100 Mbps, full duplex, flow control tx and rx
w83627hf 9191-0290: Reading VID from GPIO5
NET: Registered protocol family 10
IPv6 over IPv4 tunneling driver
audit(1154774854.770:5): avc:  denied  { write } for  pid=1360
comm="cpuspeed" name="cpufreq" dev=sysfs ino=4863
scontext=system_u:system_r:cpuspeed_t:s0
tcontext=system_u:object_r:sysfs_t:s0 tclass=dir
eth0: no IPv6 routers present
CPU0 called lock_cpu_hotplug() for app amarokapp. recursive_depth=0
 [<c01329ab>] lock_cpu_hotplug+0x36/0xb9
 [<c01182ce>] sched_getaffinity+0xf/0x83
 [<c0118361>] sys_sched_getaffinity+0x1f/0x41
 [<c0102d51>] sysenter_past_esp+0x56/0x79
amarokapp acquired cpu_bitmask_lock
CPU0 called unlock_cpu_hotplug() for app amarokapp. recursive_depth=0
 [<c0132a62>] unlock_cpu_hotplug+0x34/0xb2
 [<c0118323>] sched_getaffinity+0x64/0x83
 [<c0118361>] sys_sched_getaffinity+0x1f/0x41
 [<c0102d51>] sysenter_past_esp+0x56/0x79
amarokapp released cpu_bitmask_lock
CPU0 called lock_cpu_hotplug() for app amarokapp. recursive_depth=0
 [<c01329ab>] lock_cpu_hotplug+0x36/0xb9
 [<c01195ca>] sched_setaffinity+0xf/0xd5
 [<c01196cb>] sys_sched_setaffinity+0x3b/0x41
 [<c0102d51>] sysenter_past_esp+0x56/0x79
amarokapp acquired cpu_bitmask_lock
CPU0 called unlock_cpu_hotplug() for app amarokapp. recursive_depth=0
 [<c0132a62>] unlock_cpu_hotplug+0x34/0xb2
 [<c0119689>] sched_setaffinity+0xce/0xd5
 [<c01196cb>] sys_sched_setaffinity+0x3b/0x41
 [<c0102d51>] sysenter_past_esp+0x56/0x79
amarokapp released cpu_bitmask_lock

>
>                 Dave
>
> --
> http://www.codemonkey.org.uk
>

Regards,
Michal

-- 
Michal K. K. Piotrowski
LTG - Linux Testers Group
(http://www.stardust.webpages.pl/ltg/wiki/)
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