Re: [PATCH 0/3] posix timers: Extend kernel API to report more infoabout timers

From: Matthew Helsley
Date: Wed Feb 20 2013 - 20:21:36 EST

On Thu, Feb 14, 2013 at 8:18 AM, Pavel Emelyanov <xemul@xxxxxxxxxxxxx> wrote:
> Hi.
> I'm working on the checkpoint-restore project (, briefly
> it's aim is to collect information about process' state and saving it so
> that later it is possible to recreate the processes in the very same state
> as they were, using the collected information.
> One part of the task's state is the posix timers that this task has created.
> Currently kernel doesn't provide any API for getting information about
> what timers are currently created by process and in which state they are.
> I'd like to extend the posix timers API to provide more information about
> timers.
> Another problem with timers is the timer ID. Currently IDs are generated
> from global IDR and this makes it impossible to restore a timer from
> the saved state in general, as the required ID may be already busy at the
> time of restore.
> That said, I propose to
> 1. Change the way timer IDs are generated. This was done some time ago, so
> I'm just re-sending this patch;

Seems fine in principle. Aside: I noticed there were some
important-looking patches to the idr usage in timer id allocation

> 2. Add a system call that will list timer IDs created by the calling process;

If timers were listed in /proc like fds then you wouldn't need this
syscall. If we keep adding new syscalls like this CRIU will be
needlessly x86-specific when it could have been written more portably.

> 3. Add a system call that will allow to get the sigevent information about
> particular timer in the sigaction-like manner.

You mentioned "extending the POSIX timer API". Isn't that something
best left to standards bodies lest your changes conflict with theirs?
Again, if this were a /proc interface you wouldn't have that issue
(you'll have others ;)).

> This is actually an RFC to start discussion about how the described problems
> can be addressed. Thus, if the approach with new system calls is not acceptable,
> I'm OK to implement this in any other form.

My preference is for "other form" for the reasons above.

-Matt Helsley
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