linux-kernel - Re: [patch 3/3] timerfd: Implement write method

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Message-ID: <20140611124348.GE15786@moon>
Date:	Wed, 11 Jun 2014 16:43:48 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Andrew Vagin <avagin@...allels.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>, avagin@...nvz.org,
	xemul@...allels.com, vdavydov@...allels.com,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [patch 3/3] timerfd: Implement write method

On Wed, Jun 11, 2014 at 01:52:46PM +0400, Cyrill Gorcunov wrote:
> On Wed, Jun 11, 2014 at 01:09:15PM +0400, Andrew Vagin wrote:
> > On Wed, Jun 11, 2014 at 11:51:25AM +0400, Cyrill Gorcunov wrote:
> > > On Wed, Jun 11, 2014 at 11:27:43AM +0400, Andrew Vagin wrote:
> > 
> > Setting ticks to zero is equivalent to timerfd_read(), isn't it?
> > So do we need to re-arme the timer, if it's periodic?
> 
> I must admit I'm not really sure if we should rearm it in such
> case. In general @ticks are zeroified in case of timer-setup/cancel/read.
> 
>  - lets consider someone armed the timer it triggered but no read done
>    yet, instead ioctl called and @ticks are set to zero, then call for
>    read() and it returns zero to caller not rearming the timer (in
>    current patch approach and non-block read)
> 
>  - in turn if we rearm timer on @ticks = 0 in ioctl this makes it
>    close to behaviour of read() function (which in turn look to
>    me as a duplication of read() interface).
> 
> That said, I'm not sure yet...

What if we prohibit setting non-zero values here? @ticks are set to
zero on timerfd_setup thus there is always a way to create a timer
with fields zeroified. Something like

	case TFD_IOC_SET_TICKS: {
		u64 ticks;

		if (get_user(ticks, (u64 __user *)arg))
			return -EFAULT;
		if (!ticks)
			return -EINVAL;

		spin_lock_irq(&ctx->wqh.lock);
		if (!timerfd_canceled(ctx)) {
			ctx->ticks = ticks;
			if (ticks)
				wake_up_locked(&ctx->wqh);
			else
		} else
			ret = -ECANCELED;
		spin_unlock_irq(&ctx->wqh.lock);
		break;
	}
?
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