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 <[email protected]> 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 <[email protected]> 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 <[email protected]> > >>>> Signed-off-by: Ilias Tsitsimpis <[email protected]> > >>>> --- > >>>> 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! Thanx, Paul -- dm-devel mailing list [email protected] https://www.redhat.com/mailman/listinfo/dm-devel
