Re: [Xen-devel] [PATCH 00/12] libxl: fork: SIGCHLD flexibility

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

To check that this was doing roughly the right things, I straced xl.

Here is what it does before the first time libxl wants to fork:

 pipe([17, 18])                          = 0
 rt_sigaction(SIGCHLD, {0xb76ad507, [], SA_RESTART|SA_NOCLDSTOP}, {SIG_DFL, [], 
0}, 8) = 0
 rt_sigaction(SIGCHLD, {0xb76ad150, [], SA_RESTART|SA_NOCLDSTOP}, NULL, 8) = 0
 rt_sigaction(SIGCHLD, {0xb76ad507, [], SA_RESTART|SA_NOCLDSTOP}, NULL, 8) = 0
 clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, 
child_tidptr=0xb744a978) = 10033

That seems about right.  (It does leak the self-pipe into the child
but that is of no consequence.)


Here is what is happens in the parent when the child exits:

  --- SIGCHLD (Child exited) @ 0 (0) ---
  write(18, "\0", 1)                      = 1
  sigreturn()                             = ? (mask now [])
  gettimeofday({1389975566, 501125}, NULL) = 0
  poll([{fd=17, events=POLLIN}, {fd=7, events=POLLIN}, {fd=7, events=POLLIN}], 
3, -1) = 1 ([{fd=17, revents=POLLIN}])
  gettimeofday({1389975566, 501571}, NULL) = 0
  read(17, "\0", 256)                     = 1
  waitpid(-1, [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], WNOHANG) = 10033

That looks mostly right for the libxl child handling except that there
are two instances of {fd=7, events=POLLIN}.  I'm going to investigate
what that is and why it might be happening, starting by trying to
figure out what fd 7 is.

The trace then continues:

  close(14)                               = 0
  close(15)                               = 0
  close(12)                               = 0
  munmap(0xb766a000, 4096)                = 0
  close(11)                               = 0

I think that is the migration ao and its fds etc. being torn down.

  write(10, "\0", 1)                      = 1

This is the poller wakeup for ao completion.

  rt_sigaction(SIGCHLD, {0xb76ad150, [], SA_RESTART|SA_NOCLDSTOP}, NULL, 8) = 0
  rt_sigaction(SIGCHLD, {0xb76ad507, [], SA_RESTART|SA_NOCLDSTOP}, NULL, 8) = 0
  rt_sigaction(SIGCHLD, {SIG_DFL, [], 0}, {0xb76ad507, [], 
SA_RESTART|SA_NOCLDSTOP}, 8) = 0

And here libxl removes the ctx from the SIGCHLD users: the first two
sigaction calls are from the defer and release, and the final one
restores the application's default handler.

Ian.

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