linux-kernel - Re: [patch 14/45] posix-timers: Consolidate interval retrieval

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Date:   Sat, 01 Jul 2023 20:01:01 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        John Stultz <jstultz@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [patch 14/45] posix-timers: Consolidate interval retrieval

On Fri, Jun 30 2023 at 16:04, Frederic Weisbecker wrote:
> On Fri, Jun 30, 2023 at 03:07:17PM +0200, Thomas Gleixner wrote:
>> How exactly does this end up being copied to user space if ret != 0?
>
> kc->timer_get() doesn't return any value.
>
> So before the patch, interval is retrieved only if the target is not reaped:
>
> timer_gettime() {
>     do_timer_gettime() {
>         posix_cpu_timer_get() {
>             p = cpu_timer_task_rcu(timer);
>             if (p)
>                 itp->interval = ....
>         }
>     }
> }
>
> After the patch it's retrieved unconditionally:
>
> timer_gettime() {
>     do_timer_gettime() {
>         //unconditionally set
>         itp->interval = ....
>         posix_cpu_timer_get() {
>             p = cpu_timer_task_rcu(timer);
>             if (!p)
>                 //doesn't return any value so no failure reported

Duh. you are right ...