linux-kernel - Re: [PATCH-v2 1/9] target: Convert se_node_acl->device

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Date:	Mon, 25 May 2015 15:14:18 -0700
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	Christoph Hellwig <hch@....de>
Cc:	Christoph Hellwig <hch@...radead.org>,
	"Nicholas A. Bellinger" <nab@...erainc.com>,
	target-devel <target-devel@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Hannes Reinecke <hare@...e.de>,
	Sagi Grimberg <sagig@...lanox.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH-v2 1/9] target: Convert se_node_acl->device_list[] to
 RCU hlist

On Fri, 2015-05-22 at 13:31 +0200, Christoph Hellwig wrote:
> On Fri, May 22, 2015 at 01:55:30AM -0700, Nicholas A. Bellinger wrote:
> > > This update will now be racy, ditto for the read/write_bytes update
> > > later.
> > 
> > This should become an atomic_long_t increment, yes..?
> 
> Yes.

Converted.

> 
> > Yes, this helper is from your patch above.
> > 
> > Considering there is a single user of it here, and complexities involved
> > for a RCU conversion + bisect, is it really work adding as a separate
> > patch ahead of this one..?
> 
> The golden Linus style is to put preparatory patches first so that the
> actual logic change is as small as possible.  Adding helpers so that
> low level accesses that will e changed soon is a very typical case for that.
> 

That would be applicable here, if the patch in question had anything to
do with the actual RCU conversion itself.

Since it doesn't, I'll keep it as a separate patch after the RCU
specific changes, along with the other improvements.

> > > > +		kref_put(&orig->pr_kref, target_pr_kref_release);
> > > > +		wait_for_completion(&orig->pr_comp);
> > > >  
> > > 
> > > > +	kref_put(&orig->pr_kref, target_pr_kref_release);
> > > >  	/*
> > > > -	 * Disable struct se_dev_entry LUN ACL mapping
> > > > +	 * Before fireing off RCU callback, wait for any in process SPEC_I_PT=1
> > > > +	 * or REGISTER_AND_MOVE PR operation to complete.
> > > >  	 */
> > > > +	wait_for_completion(&orig->pr_comp);
> > > > +	kfree_rcu(orig, rcu_head);
> > > 
> > > The release callback should just call kfree_rcu, no need to wait for the
> > > release in the caller.
> > > 
> > 
> > Why doesn't se_dev_entry release this need to wait for the special case
> > references to drop..?
> 
> Why would it?

It originally had to wait in order for the se_dev_entry consumers to
drop, before it could be reused in se_node_acl->device_list[]..

> There is no access to the structure at this point, so there
> is no point to keep it around localy.  If there were other references to
> it they by defintion don't need it anymore by the time they drop the
> reference count.  Freeing a structure as soon as the refcount drops
> zero is the normal style all over the place.  Waiting for a reference
> count only makes sense if it's a drain style operation where you don't
> free the structure but you just want to wait for some class of consumers
> to stop using it.

.. but since it's not being reused anymore, doing a kfree_rcu() from the
final kref_put() should be fine.

Converting now.

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