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@xxxxxxxxxxxx>
>>> CC: Huang Ying <ying.huang@xxxxxxxxx>
>>> CC: Paul McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
>>> CC: David Howells <dhowells@xxxxxxxxxx>
>>> CC: Pranith Kumar <bobby.prani@xxxxxxxxx>
>>> CC: stable@xxxxxxxxxxxxxxx # 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;
>>>
>>
>
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