linux-kernel - Re: [PATCH 1/9] sched/core: add is

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Message-ID: <20190819085213.GA15409@gmail.com>
Date:   Mon, 19 Aug 2019 10:52:13 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Borislav Petkov <bp@...e.de>,
        Catalin Marinas <catalin.marinas@....com>,
        "David S. Miller" <davem@...emloft.net>,
        Christoph Hellwig <hch@....de>, kan.liang@...el.com,
        Peter Zijlstra <peterz@...radead.org>,
        Rik van Riel <riel@...riel.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH 1/9] sched/core: add is_kthread() helper


* Mark Rutland <mark.rutland@....com> wrote:

> On Wed, Aug 14, 2019 at 01:26:43PM +0200, Geert Uytterhoeven wrote:
> > Hi Mark,
> > 
> > On Wed, Aug 14, 2019 at 12:43 PM Mark Rutland <mark.rutland@....com> wrote:
> > > Code checking whether a task is a kthread isn't very consistent. Some
> > > code correctly tests task->flags & PF_THREAD, while other code checks
> > > task->mm (which can be true for a kthread which calls use_mm()).
> > >
> > > So that we can clean this up and keep the code easy to follow, let's add
> > > an obvious helper function to test whether a task is a kthread.
> > > Subsequent patches will use this as part of cleaning up and correcting
> > > open-coded tests.
> > >
> > > At the same time, let's fix up the kerneldoc for is_idle_task() for
> > > consistency with the new helper, using true/false rather than 0/1, given
> > > the functions return bool.
> > >
> > > Signed-off-by: Mark Rutland <mark.rutland@....com>
> > 
> > Thanks for your patch!
> > 
> > > --- a/include/linux/sched.h
> > > +++ b/include/linux/sched.h
> > > @@ -1621,13 +1621,24 @@ extern struct task_struct *idle_task(int cpu);
> > >   * is_idle_task - is the specified task an idle task?
> > >   * @p: the task in question.
> > >   *
> > > - * Return: 1 if @p is an idle task. 0 otherwise.
> > > + * Return: true if @p is an idle task, false otherwise.
> > >   */
> > >  static inline bool is_idle_task(const struct task_struct *p)
> > >  {
> > >         return !!(p->flags & PF_IDLE);
> > >  }
> > >
> > > +/**
> > > + * is_kthread - is the specified task a kthread
> > > + * @p: the task in question.
> > > + *
> > > + * Return: true if @p is a kthread, false otherwise.
> > > + */
> > > +static inline bool is_kthread(const struct task_struct *p)
> > > +{
> > > +       return !!(p->flags & PF_KTHREAD);
> > 
> > The !! is not really needed.
> > Probably you followed is_idle_task() above (where it's also not needed).
> 
> Indeed! I'm aware of the implicit bool conversion, but kept that for
> consistency.
> 
> Peter, Ingo, do you have a preference?

So the !! pattern is useful where the return value is an integer (i.e. 
there's a risk of non-bool use) - but the return value is an explicit 
bool here, so !! is IMO an entirely superfluous obfuscation.

Should probably be fixed for is_idle_task() as well?

Thanks,

	Ingo