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

[Thomas, thanks for pinging me on this.]

Hi Cyril

Please CC linux-api on changes that affect kernel-user-space ABI/API.

On Thu, May 22, 2014 at 12:35 AM, Cyrill Gorcunov [off-list ref] wrote:

On Thu, May 22, 2014 at 07:12:30AM +0900, Thomas Gleixner wrote:

quoted

There is a world outside of checkpoint/restore, really.

Yes, I simply don't know who else might use this write()
functionality for other purpose, I mean i don't see a
point to use it for anything else.

Nevertheless, when making API changes like this, we should be thinking
as generally as possible, rather than looking from the perspective of
a single use case.

quoted

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.

quoted

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

but i didn't try restore polling waiters, my bad. Letme rework this
trying addressing your comments.

Great. Please CC me and linux-api@ on the next round.

Cheers,

Michael

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Michael Kerrisk Linux man-pages maintainer;
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Author of "The Linux Programming Interface", http://blog.man7.org/