Messages in this thread | | | Subject | Re: [dm-devel] [PATCH 1/3] list_bl: Add hlist_bl_add_before/behind helpers | From | Nikos Tsironis <> | Date | Thu, 14 Mar 2019 15:28:23 +0200 |
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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
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