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

Nikos Tsironis
2019-03-20T20:25:56Z

On 3/18/19 7:16 PM, Paul E. McKenney wrote:
> On Sun, Mar 17, 2019 at 01:52:50PM +0200, Nikos Tsironis wrote:
>> On 3/14/19 5:03 PM, Paul E. McKenney wrote:
>>> On Thu, Mar 14, 2019 at 07:07:50AM -0700, Paul E. McKenney wrote:
>>>> On Thu, Mar 14, 2019 at 03:28:23PM +0200, Nikos Tsironis wrote:
>>>>> 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!
>>>>
>>>> NP, and apologies for the delay.
>>>>
>>>>>>> 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/
>>>>
>>>> Probably should keep it, then. ;-)
>>>>
>>>>>>>
>>>>>>> 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").
>>>>
>>>> Looks like I have no one to blame but myself!
>>>>
>>>> Would you like to remove that as part of your patch series?
>>>>
>>>>> 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.
>>>>
>>>> Sounds good!
>>>
>>> Oh, and of course intptr_t is one character shorter than uintptr_t, and
>>> looks to work just as well in this context. ;-)
>>>
>>> Thanx, Paul
>>>
>>
>>
>> Hi Paul,
>>
>> Sorry for the late reply.
>>
>> intptr_t seems to be defined only in a header file under arch/mips, so I
>> will stick to uintptr_t.
>
> Ah, apologies for the misdirection! Hmmm... Maybe intptr_t should be
> added alongside uintptr_t? Saving a character is saving a character. ;-)
>
> Thanx, Paul
>

I will follow up with an unrelated patch to add intptr_t to
include/linux/types.h, so that it is available in subsequent patches
throughout the kernel.

Nikos