File: restart_algorithm.txt

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This is intended as one of a series of informal documents to describe
and partially document some of the more subtle DMTCP data structures
and algorithms.  These documents are snapshots in time, and they
may become somewhat out-of-date over time (and hopefully also refreshed
to re-sync them with the code again).  (Updated Feb., 2014)

Some related documents are architecture-of-dmtcp.pdf and thread-creation.txt.

This description is primarily for i386 (32-bit Intel).  Other architectures
are simpler, and can be deduced from this description.  The sections
of this documentation are:

A. BACKGROUND
B. OVERVIEW
C. ACTUAL FUNCTION CALLS
D. HOW DOES DMTCP PATCH THE TCB


A. BACKGROUND:
    Much of the difficulty of restoring multi-threaded processes is
    in how to restore the state of glibc and the kernel.  The issues follow:
1. The run-time library, glibc, creates a TCB (thread-control-block), for
   each thread, as part of pthread_create().  That TCB specifies the
   pid and tid, which will change when a process is recreated.
2. The compiler creates thread-private variables using thread-local
   storage (TLS).  It generates references to memory at %gs:ADDR (in
   the example of i386).  This says to look up the segment descriptor
   associated with %gs, and add its base address to ADDR.  The base
   address is the beginning of the TLS.  (Hence we also call the segment
   descriptor a TLS descriptor.)
3. When the kernel does a context switch to a new thread, it must
   change %gs to point to a new segment descriptor.  Further, the kernel
   maintains the segment descriptors.  So, it must provide a kernel
   call to allow users to update those segment descriptors.  (Usually,
   this call is used only by libc.)  The call is set_thread_area (i386),
   or arch_prctl *(x86_64), or set_tls (32-bit ARM).


B. OVERVIEW:
At the time of checkpoint, writing a checkpoint image is done easily
by using the memory maps in '/proc/*/maps'.  The code for this is in
src/writeckpt.cpp, and is not discussed further here.  See
architecture-of-dmtcp.pdf for a very high-level overview, and
plugin-tutorial.pdf for how internal plugins (found in src/plugin)
are used to extend this capability.

On restart from a checkpoint image, see top of src/mtcp/mtcp_restart.c
for how the memory is restored from the checkpoint image.  Once the
memory is restored, the next tasks are:

Restore the primary stack [used by the first thread of the process]
Set:  motherpid = THREAD_REAL_TID();  [using a direct kernel call]
      [ Note that the pid and tid are the same for this first thread. ]
Patch the TCB with the correct pid and tid: TLSInfo_UpdatePID ??
Set up the segment descriptor for %gs, and restore %gs to point to it:
  TLSInfo_RestoreTLSState(motherofall);
    which calls set_thread_area
  restore gs
We may now make calls to glibc.  This is needed, since glibc makes
  a kernel call via something similar to "call *%gs:0x10", which calls
  a subroutine in the vdso segment.

Now that the first thread can make calls to glibc, it is easy for it
to recreate the other threads via clone, set_tid_address(), and
then patching the TCB and setting the segment descriptors.

When the other threads are restored, a call to setcontext will also restore
their former program counter.  So, each thread will again be in the
DMTCP signal handler.  The thread had entered the signal handler
prior to checkpoint.  The signal handler is:  threadlist.cpp:stopthisthread()

That signal handler had saved the thread registers, including its
program counter, through sigsetjmp or through getcontext, prior to
checkpoint.  It had then blocked through sem_wait().  We can now
force the new thread to call siglongjmp or setcontext to restore its
program counter and put it back in the signal handler.  The recreated
thread will again block on sem_wait(), whereupon the checkpoint thread
can release each user thread.


C. ACTUAL FUNCTION CALLS:
mtcp_restart.c calls:
threadinfo.c:Thread_PostRestart, which calls:
threadinfo.c:restoreAllThreads()
  motherpid = THREAD_REAL_TID();
  [ Do we also set motherofall->pid ? ]
  motherofall->tid = motherpid;
  TLSInfo_RestoreTLSState(motherofall);
    tlsinfo.c:TLSInfo_RestoreTLSState()
      which uses: thread->gdtentrytls[0].base_addr (for TCB)
      tlsinfo_set_thread_area (&(thread->gdtentrytls[0]))
      and tell kernel to set segment descriptor (TLS descriptor in
	GDT (global descriptor table)) of %gs for this base_addr
      If we have it right, asm("movw %gs:0x10 %0" : : m=(dummy))
	will put an address in dummy, such that *dummy points to vdso_addr
  This must be done before any libc calls, since libc wll use the
  segment descriptor of %gs to call vdso to call kernel (e.g, call *%gs:0x10)

Now that libc works, it is now safe to call clone via syscall,
to create other threads.  See the end of Section C for how each
user thread can now resume from the signal handler where it had been
suspended prior to checkpoint.


D. HOW DOES DMTCP PATCH THE TCB with the current tid and pid?
    The trick here is to know the offset into the TCB where glibc stores
the tid and pid.  This is currently found during checkpoint itself.
This could be found by looking at the include files of the glibc source
code, but they have changed from one version to another, and glibc does
not export this information.
    So currently, we take advantage of the fact that we already know the
pid and tid of the current thread.  So, we look for the bit patttern
corresponding to the adjacent pid and tid fields.  If that bit pattern
is not unique in the TCB, then we print an "internal error" message to
the user.  However, this has never occurred in our experience.
    Ideally, we would discover the offset in the TCB when libdmtcp.so
is preloaded into the application.  Any ambiguity could be discovered
within dmtcp_launch, and a second thread with a different tid could be
spawned to resolve the ambiguity.  We do not yet do this.  Note that
this should not be done within dmtcp_launch itself, since we can't be
sure which implementation of glibc (or other libc) is being used by the
end user.
    Note also, that there is no problem if we restart on a new machine
with a different version of glibc.  This is because the libc.so library
is saved and restored as part of the checkpoint image.  So, the original
version of glibc stays with the process for its lifetime (which is probably
a good thing, for the sake of robustness).