linux-kernel - Re: [PATCH 1/2] cgroup: cgroup

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Message-ID: <YaUPej4QUZoFAuOM@slm.duckdns.org>
Date:   Mon, 29 Nov 2021 07:35:54 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     lizefan.x@...edance.com, hannes@...xchg.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] cgroup: cgroup_subsys[ssid] is never NULL

On Sat, Nov 27, 2021 at 02:59:18PM +0000, Wei Yang wrote:
> When the check, (ssid) < CGROUP_SUBSYS_COUNT, passed, it means
> cgroup_subsys[ssid] is defined to its proper value. It is not
> necessary to use a true to enter the loop.
...
>  #define for_each_subsys(ss, ssid)					\
>  	for ((ssid) = 0; (ssid) < CGROUP_SUBSYS_COUNT &&		\
> -	     (((ss) = cgroup_subsys[ssid]) || true); (ssid)++)
> +	     ((ss) = cgroup_subsys[ssid]); (ssid)++)

So, now the compiler has to test whether ss is NULL or not before each
iteration even though we know that it's never NULL. The whole point of that
"|| true" is telling the compiler that the pointer is never NULL.

Thanks.

-- 
tejun