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

[Cyrill Gorcunov <gorcunov@gmail.com>]

On Thu, May 22, 2014 at 08:30:04AM +0900, Thomas Gleixner wrote:
> > 
> > > So what's the semantics of that write function? We really want to have
> > > that agreed on and documented in the man page.
> > 
> > The idea was to provide a way to setup @ticks into (nonzero) value
> > which we get from show_fdinfo output. Then when we restore it
> > we setup the timer and set @ticks to the value it had at dump
> > moment.
> 
> That's not describing the semantics. It's describing what you use it
> for.

That's I've been intending to use it for and as result the semantic
was to write unconditionally. But because I missed polling in first
place now I think such semantic is wrong and write() should be
a complete counterpart of read() method and wake up waiters.

> > > Right now the write will just update the ticks and nothing else. So
> > > what if there is a waiter already? What if there is a timer armed?
> > > 
> > > Can you please describe how checkpoint/restore is going to use all of
> > > this. How is the timer restored and how/when is the reader which was
> > > waiting in read/poll at the time of suspend reattached to it.
> > 
> > Thomas, I see what you mean. Need to think (I must admit I forgot about
> > polling of timerfds :( I were to restore timerfds like this
> > 
> >  - fetch data from fdinfo
> >  - use timer_create/settime to arm it
> >  - write @ticks then
> 
> That's clear to me.
> 
> So again you have to answer the questions:
> 
>    Do we just allow the write unconditionally?
>    Do we care about waking readers/pollers?
>  
> Whatever the answer is, it needs to be documented coherently in the
> changelog, in the code and in the man page.

"Yes" to both questions I think. Thomas I'll return with a new patchset,
testcase and man update.