Re: kref refcnt and false positives

[Greg KH <gregkh@suse.de>]

On Wed, Dec 13, 2006 at 03:34:08PM -0800, Venkatesh Pallipadi wrote:
> 
> With WARN_ON addition to kobject_init()
> [ http://kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.19/2.6.19-mm1/dont-use/broken-out/gregkh-driver-kobject-warn.patch ]
> 
> I started seeing following WARNING on CPU offline followed by online on my
> x86_64 system.
> 
> WARNING at lib/kobject.c:172 kobject_init()
> 
> Call Trace:
>  [<ffffffff8020ab45>] dump_trace+0xaa/0x3ef
>  [<ffffffff8020aec4>] show_trace+0x3a/0x50
>  [<ffffffff8020b0f6>] dump_stack+0x15/0x17
>  [<ffffffff80350abc>] kobject_init+0x3f/0x8a
>  [<ffffffff80350be1>] kobject_register+0x1a/0x3e
>  [<ffffffff803bbd89>] sysdev_register+0x5b/0xf9
>  [<ffffffff80211d0b>] mce_create_device+0x77/0xf4
>  [<ffffffff80211dc2>] mce_cpu_callback+0x3a/0xe5
>  [<ffffffff805632fd>] notifier_call_chain+0x26/0x3b
>  [<ffffffff8023f6f3>] raw_notifier_call_chain+0x9/0xb
>  [<ffffffff802519bf>] _cpu_up+0xb4/0xdc
>  [<ffffffff80251a12>] cpu_up+0x2b/0x42
>  [<ffffffff803bef00>] store_online+0x4a/0x72
>  [<ffffffff803bb6ce>] sysdev_store+0x24/0x26
>  [<ffffffff802baaa2>] sysfs_write_file+0xcf/0xfc
>  [<ffffffff8027fc6f>] vfs_write+0xae/0x154
>  [<ffffffff80280418>] sys_write+0x47/0x6f
>  [<ffffffff8020963e>] system_call+0x7e/0x83
> DWARF2 unwinder stuck at system_call+0x7e/0x83
> Leftover inexact backtrace:
> 
> This is a false positive as mce.c is unregistering/registering sysfs
> interfaces cleanly on hotplug.

The warning above tends to look like this is not true.

> kref_put() and conditional decrement of refcnt seems to be the root cause
> for this and the patch below resolves the issue for me.

Why?

Are you properly initializing your kref to null before you register it
with the driver core?  Or is it a static object?

> Original comment seemed to indicate that this conditional thing was
> performance related. Is it really? If not, we should consider the below patch.

Yes, it's a performance gain and I don't see how this patch would change
the above warning.

thanks,

greg k-h