Re: [dm-devel] [PATCH 1/3] list_bl: Add hlist_bl_add_before/behind helpers

Nikos Tsironis
2019-03-14T13:35:43Z

On 3/14/19 2:30 AM, Mike Snitzer wrote:
> On Wed, Mar 13 2019 at 7:48pm -0400,
> Paul E. McKenney <paulmck@linux.ibm.com> wrote:
>
Hi Paul,

Thanks a lot for your feedback!

>> On Thu, Feb 28, 2019 at 04:32:02PM -0500, Mike Snitzer wrote:
>>> On Thu, Dec 20 2018 at 1:06pm -0500,
>>> Nikos Tsironis <ntsironis@arrikto.com> wrote:
>>>
>>>> Add hlist_bl_add_before/behind helpers to add an element before/after an
>>>> existing element in a bl_list.
>>>>
>>>> Signed-off-by: Nikos Tsironis <ntsironis@arrikto.com>
>>>> Signed-off-by: Ilias Tsitsimpis <iliastsi@arrikto.com>
>>>> ---
>>>> include/linux/list_bl.h | 27 +++++++++++++++++++++++++++
>>>> 1 file changed, 27 insertions(+)
>>>>
>>>> diff --git a/include/linux/list_bl.h b/include/linux/list_bl.h
>>>> index 3fc2cc57ba1b..2fd918e5fd48 100644
>>>> --- a/include/linux/list_bl.h
>>>> +++ b/include/linux/list_bl.h
>>>> @@ -86,6 +86,33 @@ static inline void hlist_bl_add_head(struct hlist_bl_node *n,
>>>> hlist_bl_set_first(h, n);
>>>> }
>>>>
>>>> +static inline void hlist_bl_add_before(struct hlist_bl_node *n,
>>>> + struct hlist_bl_node *next)
>>>> +{
>>>> + struct hlist_bl_node **pprev = next->pprev;
>>>> +
>>>> + n->pprev = pprev;
>>>> + n->next = next;
>>>> + next->pprev = &n->next;
>>>> +
>>>> + /* pprev may be `first`, so be careful not to lose the lock bit */
>>>> + WRITE_ONCE(*pprev,
>>>> + (struct hlist_bl_node *)
>>>> + ((unsigned long)n |
>>>> + ((unsigned long)*pprev & LIST_BL_LOCKMASK)));
>>
>> A nit, but use of uintptr_t shrinks things a bit:
>>
>> + (struct hlist_bl_node *)
>> + ((uintptr_t)n | ((uintptr_t)*pprev & LIST_BL_LOCKMASK)));
>>
>> I am not too concerned about this, though.
>
> I'm fine with folding in your suggestion.
>

Indeed, this looks better.

>> The WRITE_ONCE() is to handle races with hlist_bl_empty() (which does contain
>> the corresponding READ_ONCE()) correct?
>
> Correct.

Yes that's correct.

>
>>>> +}
>>>> +
>>>> +static inline void hlist_bl_add_behind(struct hlist_bl_node *n,
>>>> + struct hlist_bl_node *prev)
>>>> +{
>>>> + n->next = prev->next;
>>>> + n->pprev = &prev->next;
>>>> + WRITE_ONCE(prev->next, n);
>>
>> I don't see what this WRITE_ONCE() is interacting with. The traversals
>> use plain C-language reads, and hlist_bl_empty() can't get here. All
>> uses of hlist_bl_for_each_entry() invoke hlist_bl_lock() before starting
>> the traversal, and hlist_bl_for_each_entry_safe() looks to be unused.
>> (Perhaps it should be removed? Or is there some anticipated use?)

I am using hlist_bl_for_each_entry_safe() in this proposed patch for
dm-snapshot: https://patchwork.kernel.org/patch/10835709/

>>
>> I don't believe that the WRITE_ONCE() is needed. What am I missing?
>>
>> Other than that, looks good.
>>
>> Thanx, Paul
>>
>
> I'd imagine it was just born out of symmetry with hlist_bl_add_before()
> and/or caution. But let's see what Nikos has to say.

I also don't believe that this WRITE_SAME() is needed. But, looking at
hlist_add_behind() in include/linux/list.h, which, if I am not missing
something, is used in the same way as hlist_bl_add_behind(), it also
uses WRITE_ONCE() to update prev->next:

static inline void hlist_add_behind(struct hlist_node *n,
struct hlist_node *prev)
{
n->next = prev->next;
WRITE_ONCE(prev->next, n);
n->pprev = &prev->next;

if (n->next)
n->next->pprev = &n->next;
}

Could it be the case that the WRITE_ONCE() in hlist_add_behind() is also
not needed? This WRITE_ONCE() was introduced by commit 1c97be677f72b3
("list: Use WRITE_ONCE() when adding to lists and hlists").

But, since I am not an expert in lockless programming, I opted to be on
the safe side and followed the example of hlist_add_behind().

That said, I will follow up with a new version of the patch removing the
WRITE_ONCE() and using uintptr_t instead of unsigned long.

Thanks,
Nikos