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

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Date:	Tue, 10 Jun 2014 13:05:22 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Michael Kerrisk-manpages <mtk.manpages@...il.com>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrey Vagin <avagin@...nvz.org>,
	Pavel Emelyanov <xemul@...allels.com>, vdavydov@...allels.com,
	Linux API <linux-api@...r.kernel.org>
Subject: Re: [patch 3/3] timerfd: Implement write method

On Tue, Jun 10, 2014 at 1:03 PM, Michael Kerrisk (man-pages)
<mtk.manpages@...il.com> wrote:
> [CC += linux-api@]
>
> On Tue, Jun 10, 2014 at 6:35 PM, Cyrill Gorcunov <gorcunov@...il.com> wrote:
>> On Thu, May 22, 2014 at 06:58:19AM +0900, Thomas Gleixner wrote:
>>> >
>>> > So what wakes a potential waiter in read/poll?
>>>
>>> And who is updating timerfd_create(2) ?
>>
>> Thomas, could you please take a look if the approach below is acceptable?
>> If it will be fine I update manpage then.
>> ---
>> From: Cyrill Gorcunov <gorcunov@...nvz.org>
>> Subject: timerfd: Implement timerfd_ioctl method to restore timerfd_ctx::ticks
>>
>> The read() of timerfd files allows to fetch the number of timer ticks
>> while there is no way to set it back from userspace.
>>
>> To restore the timer's state as it was at checkpoint moment we need
>> a path to bring @ticks back. Initially I thought about writing ticks
>> back via write() interface but it seems such API is somehow obscure.
>>
>> Instead implement timerfd_ioctl() method with TFD_IOC_SET_TICKS
>> command which requires CAP_SYS_RESOURCE capability to be able to
>> set @ticks into arbitrary value. Note this command doesn't wake
>> up readers/waiters and its purpose only to serve C/R needs
>> (for same sake I wrapped code with CONFIG_CHECKPOINT_RESTORE).
>> Still if needed the ioctl may be extended for new commands
>> and CONFIG_CHECKPOINT_RESTORE dropped off.

Why does this need CAP_SYS_RESOURCE?

--Andy
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