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lsusd - The Linux SUSpend Daemon

From:  NeilBrown <>
To:  Alan Stern <>, John Stultz <>, "Rafael J. Wysocki" <>, mark gross <>
Subject:  lsusd - The Linux SUSpend Daemon
Date:  Fri, 21 Oct 2011 16:23:40 +1100
Message-ID:  <20111021162340.669742c0@notabene.brown>
Cc:  Linux PM list <>, LKML <>
Archive-link:  Article


I wasn't going to do this... but then I did.  I think that sometimes coding is
a bit like chocolate.


you can find a bunch of proof-of-concept sample code that implements a
"Linux SUSpend Daemon" with client support library and test programs.

I haven't actually tested it as root and had it actually suspend and resume
and definitely haven't had it even close to a race condition, but the
various bits seem to work with each other properly when I run them under
strace and watch.

It didn't turn out quite the way I imagined, but then cold harsh reality has
a way of destroying our dreams, doesn't it :-)

Below is the README file.  Comment welcome as always.
I'm happy for patches too, but I'm equally happy for someone to re-write it
completely and make something really useful and maintainable.



This directory contains a prototype proof-of-concept system
for managing suspend in Linux.
Thus the Linux SUSpend Daemon.

It contains:

    The main daemon.  It is written to run a tight loop and blocks as
     required.  It obeys the wakeup_count protocol to get race-free
     suspend and allows clients to register to find out about
     suspend and to block it either briefly or longer term.
    It uses files in /var/run/suspend for all communication.

    File are:

      disabled:  This file always exists.  If any process holds a
        shared flock(), suspend will not happen.
      immediate:  If this file exists, lsusd will try to suspend whenever
      request:  If this is created, then lsusd will try to suspend
        once, and will remove the file when suspend completes or aborts.
      watching:  This is normally empty.  Any process wanting to know
        about suspend should take a shared flock and check the file is
        still empty, and should watch for modification.
        When suspend is imminent, lsusd creates 'watching-next', writes
         a byte to 'watching' and waits for an exclusive lock on 'watching'.
        Clients should move their lock to 'watching-next' when ready for
        When suspend completes, another byte (or 2) is written to
        'watching', and 'watching-next' is renamed over it.  Clients can
        use either of these to know that resume has happened.

      watching-next: The file that will be 'watching' in the next awake cycle.

    lsusd does not try to be event-loop based because:
      - /sys/power/wakeup_count is not pollable.  This could probably be
        'fixed' but I want code to work with today's kernel.  It will probably
        only block 100msec at most, but that might be too long???
      - I cannot get an event notification when a lock is removed from a
        file. :-(  And I think locks are an excellent light-weight
        mechanism for blocking suspend.

      This is an event-loop based daemon that can therefore easily handle
      socket connections and client protocols which need prompt
      response.  It communicates with lsusd and provides extra
      services to client.

      lsused (which needs a better name) listens on the socket
      A client may connect and send a list of file descriptors.
      When a suspend is immanent, if any file descriptor is readable,
      lsused will send a 'S' message to the client and await an 'R' reply
      (S == suspend, R == ready).  When all replies are in, lsused will
      allow the suspend to complete.  When it does (or aborts), it will send
      'A' (awake) to those clients to which it sent 'S'.

      This allows a client to get a chance to handle any wakeup events,
      but not to be woken unnecessarily on every suspend.

      This allows clients to register on the socket.
      They write a timestamp in seconds since epoch, and will receive
      a 'Now' message when that time arrives.
      Between the time the connection is made and the time a "seconds"
      number is written, suspend will be blocked.
      Also between the time that "Now" is sent and when the socket is
      closed, suspend is also blocked.

      A simple tool to create the 'request' file and then wait for it
      to be removed.

   libsus.a:  A library of client-side interfaces.
      suspend_open, suspend_block, suspend_allow, suspend_close:
           easy interface to blocking suspend
      suspend_watch, suspend_unwatch:
           For use in libevent program to get notifications of
           suspend and resume via the 'watching' file.
      wake_set, wake_destory:
           create a libevent event for an fd which is protected from
           suspend. Whenever it is readable, suspend will not be entered.
      wakealarm_set, wakealarm_destroy:
           create a libevent event for a particular time which will
           trigger even if system is suspend, and will protect against
           suspend while event is happening.

   block_test watch_test event_test alarm_test
        simple test programs for the above interfaces.
       Sample code for detecting suspend/resume from python
       Sample code for disabling suspend from shell.

All code is available under GPLv2+.  However if you ask for a different
license I am unlikely to refuse (at least with the early prototype).

Patches and comment are welcome, but please also feel free to include
any of this in some more complete framework.

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