kref refcnt and false positives

[Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>]

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.

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

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

Thanks,
Venki





Now that kobject_init has a WARN_ON for refcnt, change below is needed
to avoid false positives.

Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>

Index: linux-2.6.19-rc-mm/lib/kref.c
===================================================================
--- linux-2.6.19-rc-mm.orig/lib/kref.c
+++ linux-2.6.19-rc-mm/lib/kref.c
@@ -52,12 +52,7 @@ int kref_put(struct kref *kref, void (*r
 	WARN_ON(release == NULL);
 	WARN_ON(release == (void (*)(struct kref *))kfree);
 
-	/*
-	 * if current count is one, we are the last user and can release object
-	 * right now, avoiding an atomic operation on 'refcount'
-	 */
-	if ((atomic_read(&kref->refcount) == 1) ||
-	    (atomic_dec_and_test(&kref->refcount))) {
+	if (atomic_dec_and_test(&kref->refcount)) {
 		release(kref);
 		return 1;
 	}