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

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Message-ID: <20151020004108.GB27292@linux-uzut.site>
Date:	Mon, 19 Oct 2015 17:41:08 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Frederic Weisbecker <fweisbec@...il.com>
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 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_*

[...]

>@@ -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?

Thanks,
Davidlohr
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