As I mentioned earlier, I don't know what sort of approach we want
to take to guide checkpoint and restart.  I.e. do we want it to be
a mostly userspace-orchestrated affair, or entirely done in the
kernel using the freezer or some other mechanism in response to a
single syscall or containerfs file write?

If we wanted to do a lot of the work in userspace, here is a pair of
patches to read and restore signal information.  It's entirely unsafe
wrt locking, bc i would assume that if we did in fact do c/r from
userspace, we would have some way of entirely pulling the task off
the runqueue while doing our thing...

Anyway, this is purely to start discussion.

thanks,
-serge

>From 30bbe322942e5ed86bad63861dad80595cd04063 Mon Sep 17 00:00:00 2001
From: Serge E. Hallyn <serue@us.ibm.com>
Date: Mon, 30 Apr 2007 16:22:44 -0400
Subject: [PATCH 1/2] signal checkpoint: define /proc/pid/sig/

Define /proc/<pid>/sig/ directory containing files to report
on a process' signal info.

Files defined:
	action: list signal action
	altstack: print sigaltstack location and size
	blocked: print blocked signal mask
	pending: print pending signals and siginfo
	shared_pending: print shared pending signals and siginfo
	waiters: list tasks wait4()ing on task PID.

TESTING:
In one terminal run 'forker' as compiled from forker.c below.  In
another terminal, run 'sh checkforker.sh' also included below
which will send more signals to the forker, then check it's pending,
blocked, and shared_pending sig/ files.

=====================================================================
forker.c
=====================================================================

void runchild(void)
{
	printf("child exiting\n");
	exit(0);
}

int main()
{
	sigset_t blockset;
	sigset_t prevset;
	int pid;

	if (sigemptyset(&blockset) == -1) {
		perror("sigemptyset");
		return -1;
	}

	if (sigaddset(&blockset, SIGUSR1) == -1) {
		perror("sigaddset");
		return -1;
	}

	if (sigaddset(&blockset, SIGUSR2) == -1) {
		perror("sigaddset");
		return -1;
	}

	if (sigaddset(&blockset, SIGCHLD) == -1) {
		perror("sigaddset");
		return -1;
	}

	if (sigaddset(&blockset, SIGSEGV) == -1) {
		perror("sigaddset");
		return -1;
	}

	if (sigaddset(&blockset, SIGPOLL) == -1) {
		perror("sigaddset");
		return -1;
	}

	if (sigprocmask(SIG_SETMASK, &blockset, &prevset) == -1) {
		perror("sigprocmask");
		return -1;
	}

	pid = fork();
	if (pid < 0)
		perror("fork");
	if (pid == 0)
		runchild();
	sleep(30);
}
=====================================================================

=====================================================================
checkforker.sh
=====================================================================
p=`ps -ef | grep forker | head -1 | awk '{ print $2 }'`
kill -10 $p
kill -29 $p
kill -11 $p
echo pending
cat /proc/$p/sig/pending
echo blocked
cat /proc/$p/sig/blocked
echo shared pending
cat /proc/$p/sig/shared_pending
=====================================================================

Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
---
 Documentation/filesystems/proc.txt |   59 +++++++++--
 fs/proc/base.c                     |   62 +++++++++++
 include/linux/signal.h             |    1 +
 kernel/signal.c                    |  202 ++++++++++++++++++++++++++++++++++++
 4 files changed, 312 insertions(+), 12 deletions(-)

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index 8756a07..1d74e19 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -133,6 +133,7 @@ Table 1-1: Process specific entries in /proc
  maps		Memory maps to executables and library files	(2.4)
  mem		Memory held by this process
  root		Link to the root directory of this process
+ sig     Directory, which contains signal information
  stat		Process status
  statm		Process memory status information
  status		Process status in human readable form
@@ -188,16 +189,50 @@ Table 1-2: Contents of the statm files (as of 2.6.8-rc3)
  dt       number of dirty pages			(always 0 on 2.6)
 ..............................................................................
 
+Table 1-3: Contents of sig directory files
+..............................................................................
+ File             Content
+ action           List sigaction as "signum flags sigmask handler"
+ altstack         List location and size of sigaltstack
+ blocked          For each signal, print '1' if blocked, '0' otherwise
+ pending          For each signal, print '1' if pending for this thread,
+                  '0' otherwise
+                  Also print additional information for certain given
+                  pending signals.  See 'pending' entry below for details.
+ shared_pending   For each signal, print '1' if pending for all threads,
+                  '0' otherwise.
+ waiters          For each task doing wait on task PID, list the flags and
+                  the waiting process' pid.
+..............................................................................
+
+The /proc/PID/sig/pending file prints additional pending signal information
+depending on the signal.  See Table 1-4 for details by signal number.
+
+Table 1-4: Details of /proc/PID/sig/pending info for pending signals
+..............................................................................
+ Signal     Information printed
+ SIGKILL    Pid and uid of killing process
+
+ SIGCHLD    Pid, uid, status, utime, and stime of exited child process
+
+ SIGILL
+ SIGFPE
+ SIGSEGV
+ SIGBUS     Address and (if applicable to arch) trap number
+
+ SIGPOLL    Band and fd
+..............................................................................
+
 1.2 Kernel data
 ---------------
 
 Similar to  the  process entries, the kernel data files give information about
 the running kernel. The files used to obtain this information are contained in
-/proc and  are  listed  in Table 1-3. Not all of these will be present in your
+/proc and  are  listed  in Table 1-5. Not all of these will be present in your
 system. It  depends  on the kernel configuration and the loaded modules, which
 files are there, and which are missing.
 
-Table 1-3: Kernel info in /proc 
+Table 1-5: Kernel info in /proc 
 ..............................................................................
  File        Content                                           
  apm         Advanced power management info                    
@@ -476,7 +511,7 @@ subdirectories. These  are  named  ide0,  ide1  and  so  on.  Each  of  these
 directories contains the files shown in table 1-4.
 
 
-Table 1-4: IDE controller info in  /proc/ide/ide? 
+Table 1-6: IDE controller info in  /proc/ide/ide? 
 ..............................................................................
  File    Content                                 
  channel IDE channel (0 or 1)                    
@@ -490,7 +525,7 @@ controllers directory.  The  files  listed in table 1-5 are contained in these
 directories.
 
 
-Table 1-5: IDE device information 
+Table 1-7: IDE device information 
 ..............................................................................
  File             Content                                    
  cache            The cache                                  
@@ -532,12 +567,12 @@ the drive parameters:
 1.4 Networking info in /proc/net
 --------------------------------
 
-The subdirectory  /proc/net  follows  the  usual  pattern. Table 1-6 shows the
+The subdirectory  /proc/net  follows  the  usual  pattern. Table 1-8 shows the
 additional values  you  get  for  IP  version 6 if you configure the kernel to
-support this. Table 1-7 lists the files and their meaning.
+support this. Table 1-9 lists the files and their meaning.
 
 
-Table 1-6: IPv6 info in /proc/net 
+Table 1-8: IPv6 info in /proc/net 
 ..............................................................................
  File       Content                                               
  udp6       UDP sockets (IPv6)                                    
@@ -552,7 +587,7 @@ Table 1-6: IPv6 info in /proc/net
 ..............................................................................
 
 
-Table 1-7: Network info in /proc/net 
+Table 1-9: Network info in /proc/net 
 ..............................................................................
  File          Content                                                         
  arp           Kernel  ARP table                                               
@@ -676,10 +711,10 @@ The directory  /proc/parport  contains information about the parallel ports of
 your system.  It  has  one  subdirectory  for  each port, named after the port
 number (0,1,2,...).
 
-These directories contain the four files shown in Table 1-8.
+These directories contain the four files shown in Table 1-10.
 
 
-Table 1-8: Files in /proc/parport 
+Table 1-10: Files in /proc/parport 
 ..............................................................................
  File      Content                                                             
  autoprobe Any IEEE-1284 device ID information that has been acquired.         
@@ -697,10 +732,10 @@ Table 1-8: Files in /proc/parport
 
 Information about  the  available  and actually used tty's can be found in the
 directory /proc/tty.You'll  find  entries  for drivers and line disciplines in
-this directory, as shown in Table 1-9.
+this directory, as shown in Table 1-11.
 
 
-Table 1-9: Files in /proc/tty 
+Table 1-11: Files in /proc/tty 
 ..............................................................................
  File          Content                                        
  drivers       list of drivers and their usage                
diff --git a/fs/proc/base.c b/fs/proc/base.c
index a5fa1fd..8e0dd7a 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@