linux-kernel - Re: [PATCH] llist: Fix missing lockless

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Message-ID: <20150210163810.GP4166@linux.vnet.ibm.com>
Date:	Tue, 10 Feb 2015 08:38:10 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Pranith Kumar <bobby.prani@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Huang Ying <ying.huang@...el.com>,
	linux-kernel@...r.kernel.org, David Howells <dhowells@...hat.com>,
	stable@...r.kernel.org
Subject: Re: [PATCH] llist: Fix missing lockless_dereference()

On Tue, Feb 10, 2015 at 09:03:50AM -0500, Peter Hurley wrote:
> On 02/06/2015 09:08 PM, Mathieu Desnoyers wrote:
> > A lockless_dereference() appears to be missing in llist_del_first().
> > It should only matter for Alpha in practice.
> > 
> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > CC: Huang Ying <ying.huang@...el.com>
> > CC: Paul McKenney <paulmck@...ux.vnet.ibm.com>
> > CC: David Howells <dhowells@...hat.com>
> > CC: Pranith Kumar <bobby.prani@...il.com>
> > CC: stable@...r.kernel.org # v3.1+
> > ---
> >  lib/llist.c | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> > 
> > diff --git a/lib/llist.c b/lib/llist.c
> > index f76196d..f34e176 100644
> > --- a/lib/llist.c
> > +++ b/lib/llist.c
> > @@ -26,6 +26,7 @@
> >  #include <linux/export.h>
> >  #include <linux/interrupt.h>
> >  #include <linux/llist.h>
> > +#include <linux/rcupdate.h>
> 
> Pranith,
> 
> I didn't realize you put lockless_dereference() in rcupdate.h
> 
> If the point of lockless_reference() is to provide a utility function for
> situations _not_ involving RCU, then it doesn't make sense to provide it
> in an RCU header file.

OK, I'll bite.  Just where do you suggest putting it?  ;-)

That question aside, lockless_dereference() does resemble the
rcu_dereference() family of APIs.  This of course means that having it in
rcupdate.h near rcu_dereference() makes it easier to maintain, given that
needed changes to one are likely to require at least review of the rest.

							Thanx, Paul

> Regards,
> Peter Hurley
> 
> PS - That's not an objection to this patch, though.
> 
> >  /**
> > @@ -67,7 +68,12 @@ struct llist_node *llist_del_first(struct llist_head *head)
> >  {
> >  	struct llist_node *entry, *old_entry, *next;
> >  
> > -	entry = head->first;
> > +	/*
> > +	 * Load entry before entry->next. Matches the implicit
> > +	 * memory barrier before the cmpxchg in llist_add_batch(),
> > +	 * which ensures entry->next is stored before entry.
> > +	 */
> > +	entry = lockless_dereference(head->first);
> >  	for (;;) {
> >  		if (entry == NULL)
> >  			return NULL;
> > 
> 

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