linux-kernel - Re: [PATCH v3 1/3] msi: free msi_desc entry only after we've released the kobject

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Date:	Tue, 29 Oct 2013 14:49:53 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Veaceslav Falico <vfalico@...hat.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Yinghai Lu <yinghai@...nel.org>,
	Knut Petersen <Knut_Petersen@...nline.de>,
	Ingo Molnar <mingo@...nel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Neil Horman <nhorman@...driver.com>
Subject: Re: [PATCH v3 1/3] msi: free msi_desc entry only after we've
 released the kobject

On Tue, Oct 29, 2013 at 09:34:28AM -0700, Linus Torvalds wrote:
> On Tue, Oct 29, 2013 at 3:30 AM, Veaceslav Falico <vfalico@...hat.com> wrote:
> >                 /*
> >                  * Its possible that we get into this path
> >                  * When populate_msi_sysfs fails, which means the entries
> >                  * were not registered with sysfs.  In that case don't
> > -                * unregister them.
> > +                * unregister them, and just free. Otherwise the
> > +                * kobject->release will take care of freeing the entry via
> > +                * msi_kobj_release().
> >                  */
> >                 if (entry->kobj.parent) {
> >                         kobject_del(&entry->kobj);
> >                         kobject_put(&entry->kobj);
> > +               } else {
> > +                       kfree(entry);
> >                 }
> > -
> > -               list_del(&entry->list);
> > -               kfree(entry);
> 
> So this code sequence still makes me very unhappy.
> 
> Why does not just a simple unconditional
> 
>         kobject_del(&entry->kobj);
>         kobject_put(&entry->kobj);
> 
> work for the "not registered with sysfs" case? And if the sysfs code
> really gets confused, why not
> 
>         if (entry->kobj.parent)
>                 kobject_del(&entry->kobj);
>         kobject_put(&entry->kobj);
> 
> (btw, looking at the sysfs code, this looks *very* suspicious in
> sysfs_remove_dir():
> 
>         struct sysfs_dirent *sd = kobj->sd;
> 
>         spin_lock(&sysfs_assoc_lock);
>         kobj->sd = NULL;
>         spin_unlock(&sysfs_assoc_lock);
> 
> and I would suggest that "sd = kobj->sd" should be done under the
> lock, because otherwise the lock is kind of pointless..)
> 
> Greg?

That is really odd, but I guess it works as-is because no one ever calls
that function on the same kobject at the same time.  I don't know what
that is trying to do.  There has been some work by Tejun in this area
for linux-next, but that lock and logic is still there, I'll look into
fixing that up...

thanks,

greg k-h
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