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

[Peter Hurley <peter@hurleysoftware.com>]

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.

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;
>>>
>>
>