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

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Date:	Wed, 11 Jun 2014 13:52:46 +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 01:09:15PM +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:
> 
> ...
> 
> > +#ifdef CONFIG_CHECKPOINT_RESTORE
> > +static long timerfd_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
> > +{
> > +	struct timerfd_ctx *ctx = file->private_data;
> > +	int ret = 0;
> > +
> > +	switch (cmd) {
> > +	case TFD_IOC_SET_TICKS: {
> > +		u64 ticks;
> > +
> > +		if (get_user(ticks, (u64 __user *)arg))
> > +			return -EFAULT;
> > +		spin_lock_irq(&ctx->wqh.lock);
> > +		ctx->ticks = ticks;
> > +		if (ticks)
> > +			wake_up_locked(&ctx->wqh);
> 
> 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...
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