Messages in this thread | | | Date | Sun, 24 Jul 2016 15:43:33 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] list: Expand list_first_entry_or_null() |
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On Sat, Jul 23, 2016 at 07:27:50PM +0100, Chris Wilson wrote: > Due to the use of READ_ONCE() in list_empty() the compiler cannot > optimise !list_empty() ? list_first_entry() : NULL very well. By > manually expanding list_first_entry_or_null() we can take advantage of > the READ_ONCE() to avoid the list element changing under the test while > the compiler can generate smaller code. > > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> > Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> > Cc: Andrew Morton <akpm@linux-foundation.org> > Cc: Dan Williams <dan.j.williams@intel.com> > Cc: Jan Kara <jack@suse.cz> > Cc: Josef Bacik <jbacik@fb.com> > Cc: linux-kernel@vger.kernel.org
Queued for review and testing, thank you!
Thanx, Paul
> --- > include/linux/list.h | 7 +++++-- > 1 file changed, 5 insertions(+), 2 deletions(-) > > diff --git a/include/linux/list.h b/include/linux/list.h > index 5356f4d661a7..7f8b08492cb3 100644 > --- a/include/linux/list.h > +++ b/include/linux/list.h > @@ -381,8 +381,11 @@ static inline void list_splice_tail_init(struct list_head *list, > * > * Note that if the list is empty, it returns NULL. > */ > -#define list_first_entry_or_null(ptr, type, member) \ > - (!list_empty(ptr) ? list_first_entry(ptr, type, member) : NULL) > +#define list_first_entry_or_null(ptr, type, member) ({ \ > + struct list_head *head__ = (ptr); \ > + struct list_head *pos__ = READ_ONCE(head__->next); \ > + pos__ != head__ ? list_entry(pos__, type, member) : NULL; \ > +}) > > /** > * list_next_entry - get the next element in list > -- > 2.8.1 >
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