Re: [patch 3/3] timerfd: Implement write method

On Tue, Jun 10, 2014 at 01:05:22PM -0700, Andy Lutomirski wrote:

On Tue, Jun 10, 2014 at 1:03 PM, Michael Kerrisk (man-pages)
[off-list ref] wrote:

quoted

[CC += linux-api@]

Thanks Michael!

quoted

On Tue, Jun 10, 2014 at 6:35 PM, Cyrill Gorcunov [off-list ref] wrote:

quoted

On Thu, May 22, 2014 at 06:58:19AM +0900, Thomas Gleixner wrote:

quoted

quoted

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.
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From: Cyrill Gorcunov <redacted>
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?

  Because I think this interface should not be used by a regular
applications, the only purpose is to restore the @ticks after
checkpoint. Requiring CAP_SYS_RESOURCE means that at least
program which use it knows what it's doing.

  Still if someone has a scenarion where we might need this
intarface out of this cap requirement -- we always can
drop it of without breaking existing users, but not the
reverse.

P.S. I remember Thomas' words about existence of the other
word out of c/r, still I treat this ioctl as exception
(as in prctl codes we use for c/r).