Re: [PATCH] llist: Fix missing lockless_dereference()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Feb 10, 2015 at 12:29:24PM -0500, Peter Hurley wrote:
> On 02/10/2015 11:38 AM, Paul E. McKenney wrote:
> > 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@efficios.com>
> >>> CC: Huang Ying <ying.huang@intel.com>
> >>> CC: Paul McKenney <paulmck@linux.vnet.ibm.com>
> >>> CC: David Howells <dhowells@redhat.com>
> >>> CC: Pranith Kumar <bobby.prani@gmail.com>
> >>> CC: stable@vger.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?  ;-)
> 
> Two possibilities:
> 1. linux/compiler.h where READ/WRITE/ACCESS_ONCE() are, or
> 2. a new arch-independent header sucked in by asm/barrier.h (because it's
>    basically a barrier abstraction, in the same way that smp_load_acquire/
>    smp_store_release are)
> 
> 
> > 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.
> 
> I can understand how and why it got there.
> But it's not an RCU abstraction, so having random users pulling in RCU headers
> to get at a convenient (but not strictly necessary) helper function is less than
> ideal.
> 
> Honestly, I'd rather see the naked smp_read_barrier_depends() than wondering why
> someone grabbed linux/rcupdate.h for the lockless list implementation.

The usual fix for this problem is to list the API member as a comment
at the end of the #include line.

							Thanx, Paul

> Regards,
> Peter Hurley
> 
> 
> >>>  /**
> >>> @@ -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;
> >>>
> >>
> > 
>