Re: [PATCH] ipc/sem: Three function calls less in do_semtimedop()

Al Viro
2019-07-06T20:50:11Z

On Sat, Jul 06, 2019 at 09:13:46PM +0100, Colin Ian King wrote:
> On 06/07/2019 13:28, Markus Elfring wrote:
> > From: Markus Elfring <elfring@users.sourceforge.net>
> > Date: Sat, 6 Jul 2019 14:16:24 +0200
> >
> > Avoid three function calls by using ternary operators instead of
> > conditional statements.
> >
> > This issue was detected by using the Coccinelle software.
> >
> > Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
> > ---
> > ipc/sem.c | 25 ++++++++-----------------
> > 1 file changed, 8 insertions(+), 17 deletions(-)
> >
> > diff --git a/ipc/sem.c b/ipc/sem.c
> > index 7da4504bcc7c..56ea549ac270 100644
> > --- a/ipc/sem.c
> > +++ b/ipc/sem.c
> > @@ -2122,27 +2122,18 @@ static long do_semtimedop(int semid, struct sembuf __user *tsops,
> > int idx = array_index_nospec(sops->sem_num, sma->sem_nsems);
> > curr = &sma->sems[idx];
> >
> > - if (alter) {
> > - if (sma->complex_count) {
> > - list_add_tail(&queue.list,
> > - &sma->pending_alter);
> > - } else {
> > -
> > - list_add_tail(&queue.list,
> > - &curr->pending_alter);
> > - }
> > - } else {
> > - list_add_tail(&queue.list, &curr->pending_const);
> > - }
> > + list_add_tail(&queue.list,
> > + alter
> > + ? (sma->complex_count
> > + ? &sma->pending_alter
> > + : &curr->pending_alter)
> > + : &curr->pending_const);
>
> Just no. This is making the code harder to comprehend with no advantage.
> Compilers are smart, let the do the optimization work and keep code
> simple for us mere mortals.

If anything, that would've been better off as

int idx = array_index_nospec(sops->sem_num, sma->sem_nsems);
struct sem *curr = &sma->sems[idx];
struct list_head *list; /* which queue to sleep on */

if (!alter)
list = &curr->pending_const;
else if (sma->complex_count)
list = &sma->pending_alter;
else
list = &curr->pending_alter;

list_add_tail(&queue.list, list);

perhaps with better identifier than 'list'. This kind of ?: (ab)use makes
for unreadable code and more than makes up for "hey, we are adding to some
list in all those cases" extra information passed to readers...