From: Mike Snitzer <snitzer@redhat.com>
To: "Paul E. McKenney" <paulmck@linux.ibm.com>,
Nikos Tsironis <ntsironis@arrikto.com>
Cc: hch@infradead.org, agk@redhat.com, dm-devel@redhat.com,
mpatocka@redhat.com, iliastsi@arrikto.com,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3] list_bl: Add hlist_bl_add_before/behind helpers
Date: Wed, 13 Mar 2019 20:30:28 -0400 [thread overview]
Message-ID: <20190314003027.GE4202@redhat.com> (raw)
In-Reply-To: <20190313234853.GA7797@linux.ibm.com>
On Wed, Mar 13 2019 at 7:48pm -0400,
Paul E. McKenney <paulmck@linux.ibm.com> wrote:
> 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.
> The WRITE_ONCE() is to handle races with hlist_bl_empty() (which does contain
> the corresponding READ_ONCE()) correct?
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 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.
Thanks,
Mike
next prev parent reply other threads:[~2019-03-14 0:30 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20181220180651.4879-1-ntsironis@arrikto.com>
[not found] ` <20181220180651.4879-2-ntsironis@arrikto.com>
2019-02-28 21:32 ` [PATCH 1/3] list_bl: Add hlist_bl_add_before/behind helpers Mike Snitzer
2019-02-28 21:34 ` Mike Snitzer
2019-03-11 18:16 ` Christoph Hellwig
2019-03-11 22:13 ` Paul E. McKenney
2019-03-11 22:43 ` Mike Snitzer
2019-03-14 0:25 ` Paul E. McKenney
2019-03-13 23:48 ` Paul E. McKenney
2019-03-14 0:30 ` Mike Snitzer [this message]
2019-03-14 13:28 ` [dm-devel] " Nikos Tsironis
2019-03-14 14:07 ` Paul E. McKenney
2019-03-14 15:03 ` Paul E. McKenney
2019-03-17 11:52 ` Nikos Tsironis
2019-03-18 17:16 ` Paul E. McKenney
2019-03-20 20:25 ` Nikos Tsironis
2019-03-14 17:01 ` Nikos Tsironis
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