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

From: Nikos Tsironis <ntsironis@arrikto.com>
To: paulmck@linux.ibm.com
Cc: Mike Snitzer <snitzer@redhat.com>,
	hch@infradead.org, agk@redhat.com, dm-devel@redhat.com,
	mpatocka@redhat.com, iliastsi@arrikto.com,
	linux-kernel@vger.kernel.org
Subject: Re: [dm-devel] [PATCH 1/3] list_bl: Add hlist_bl_add_before/behind helpers
Date: Sun, 17 Mar 2019 13:52:50 +0200	[thread overview]
Message-ID: <ad704078-1a28-5726-c428-7be19dbe2466@arrikto.com> (raw)
In-Reply-To: <20190314150306.GA22051@linux.ibm.com>

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.

Thanks,
Nikos