linux-kernel - Re: [PATCH] posix-cpu-timers: Merge running and checking

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Date:	Tue, 20 Oct 2015 02:55:36 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	George Spelvin <linux@...izon.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jason Low <jason.low2@...com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] posix-cpu-timers: Merge running and checking_timer state
 in one field

On Mon, Oct 19, 2015 at 05:41:08PM -0700, Davidlohr Bueso wrote:
> On Tue, 20 Oct 2015, Frederic Weisbecker wrote:
> 
> >- * @checking_timer:	true when a thread in the group is in the
> >- *			process of checking for thread group timers.
> >- *
> >+ * @state:		flags describing the current state of the cputimer.
> >+ *			CPUTIMER_STATE_RUNNING bit means the timers is elapsing.
> 
>                                                              s/timers/timer
> 
> >+ * 			CPUTIMER_STATE_CHECKING bit means that the cputimer has
> >+ *			expired and a thread in the group is checking the
> >+ *			callback list.
> 
> These comments might be better served when defining CPUTIMER_STATE_*

If it was defined as an emum I'd agree but here state is defined as an
int (whose size is more readable in a struct than enum) and it's not obvious
what kind of values it can take if we don't define them here.

> 
> [...]
> 
> >@@ -606,7 +606,7 @@ bool posix_cpu_timers_can_stop_tick(struct task_struct *tsk)
> >		return false;
> >
> >	/* Check if cputimer is running. This is accessed without locking. */
> >-	if (READ_ONCE(tsk->signal->cputimer.running))
> >+	if (READ_ONCE(tsk->signal->cputimer.state))
> >		return false;
> 
> Could we have cases, such as the above, where .state is set to CPUTIMER_STATE_CHECKING
> and therefore the check is not equivalent?

Nope we shouldn't. I added a WARN_ONCE somewhere to perform some related sanity
checks. I could add more if needed.

Thanks.

> Thanks,
> Davidlohr
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