linux-kernel - Re: [PATCH] ipc/sem: Three function calls less in do

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Message-ID: <20190706205003.GP17978@ZenIV.linux.org.uk>
Date:   Sat, 6 Jul 2019 21:50:03 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Colin Ian King <colin.king@...onical.com>
Cc:     Markus Elfring <Markus.Elfring@....de>,
        kernel-janitors@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Manfred Spraul <manfred@...orfullife.com>,
        Mathieu Malaterre <malat@...ian.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ipc/sem: Three function calls less in do_semtimedop()

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@...rs.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@...rs.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...