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Date:   Mon, 18 Mar 2019 10:16:40 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Nikos Tsironis <ntsironis@...ikto.com>
Cc:     Mike Snitzer <snitzer@...hat.com>, hch@...radead.org,
        agk@...hat.com, dm-devel@...hat.com, mpatocka@...hat.com,
        iliastsi@...ikto.com, linux-kernel@...r.kernel.org
Subject: Re: [dm-devel] [PATCH 1/3] list_bl: Add hlist_bl_add_before/behind
 helpers

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@...ux.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@...ikto.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@...ikto.com>
> >>>>>>> Signed-off-by: Ilias Tsitsimpis <iliastsi@...ikto.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

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