Re: [PATCH 3/N] remote follow fork and spurious child stops in non-stop mode

[Pedro Alves <palves@redhat.com>]

On 07/29/2015 02:21 PM, Pedro Alves wrote:

> 
> I tried writing a test for this, by making a multithreaded program
> have all its threads but the main continuously fork (see attached), while
> the main thread continuously steps over a breakpoint (a conditional
> breakpoint with condition "0" should do it, as gdbserver handles
> that breakpoint itself), but that stumbles on yet more problems...  :-/
> 
> $ ./gdb ./testsuite/gdb.threads/fork-plus-threads-2 -ex "set non-stop on" -ex "set detach-on-fork off" -ex "tar extended-rem :9999"
> ...
> Remote debugging using :9999
> (gdb)
> [Thread 24971.24971] #1 stopped.
> 0x0000003615a011f0 in ?? ()
> c&
> Continuing.
> (gdb) [New Thread 24971.24981]
> [New Thread 24983.24983]
> [New Thread 24971.24982]
> 
> [Thread 24983.24983] #3 stopped.
> 0x0000003615ebc7cc in __libc_fork () at ../nptl/sysdeps/unix/sysv/linux/fork.c:130
> 130       pid = ARCH_FORK ();
> [New Thread 24984.24984]
> Error in re-setting breakpoint -16: PC register is not available
> Error in re-setting breakpoint -17: PC register is not available
> Error in re-setting breakpoint -18: PC register is not available
> Error in re-setting breakpoint -19: PC register is not available
> Error in re-setting breakpoint -24: PC register is not available
> Error in re-setting breakpoint -25: PC register is not available
> Error in re-setting breakpoint -26: PC register is not available
> Error in re-setting breakpoint -27: PC register is not available
> Error in re-setting breakpoint -28: PC register is not available
> Error in re-setting breakpoint -29: PC register is not available
> Error in re-setting breakpoint -30: PC register is not available
> PC register is not available
> (gdb)
> 

Hmm, gdbserver's logs (for a different run) show:

...
HEW: Got clone event from LWP 25962, new child is LWP 25989
my_waitpid (-1, 0x40000001)
my_waitpid (-1, 0x1): status(137f), 25990
LWFE: waitpid(-1, ...) returned 25990, ERRNO-OK
LLW: waitpid 25990 received Stopped (signal) (stopped)
my_waitpid (-1, 0x40000001)
my_waitpid (-1, 0x1): status(0), 25988
LWFE: waitpid(-1, ...) returned 25988, ERRNO-OK
LLW: waitpid 25988 received 0 (exited)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
LLFE: 25988 exited.
^^^^^^^^^^^^^^^^^^^
my_waitpid (-1, 0x40000001)
my_waitpid (-1, 0x80000001): status(1057f), 25973
LWFE: waitpid(-1, ...) returned 25973, ERRNO-OK
LLW: waitpid 25973 received Trace/breakpoint trap (stopped)
pc is 0x3615ebc7cc
HEW: Got fork event from LWP 25973, new child is 25990
pc is 0x3615ebc7cc
pc is 0x3615ebc7cc
my_waitpid (-1, 0x40000001)
my_waitpid (-1, 0x80000001): status(117f), 25972
LWFE: waitpid(-1, ...) returned 25972, ERRNO-OK
LLW: waitpid 25972 received Child exited (stopped)
pc is 0x3616a0f279
my_waitpid (-1, 0x40000001)
my_waitpid (-1, 0x80000001): status(117f), 0
LWFE: waitpid(-1, ...) returned 0, ERRNO-OK
RSRL: resuming stopped-resumed LWP LWP 25962.25962 at 3615ef4ce1: step=0
pc is 0x3615ef4ce1
Resuming lwp 25962 (continue, signal 0, stop not expected)
  continue from pc 0x3615ef4ce1
RSRL: resuming stopped-resumed LWP LWP 25962.25989 at 0: step=0
pc is 0x3615ef4ce1
Resuming lwp 25989 (continue, signal 0, stop not expected)
  continue from pc 0x3615ef4ce1
sigchld_handler
Ignored signal 17 for LWP 25972.
pc is 0x3616a0f279
Resuming lwp 25972 (continue, signal 17, stop not expected)
  continue from pc 0x3616a0f279
handling possible target event
>>>> entering linux_wait_1
linux_wait_1: [<all threads>]
Got a pending child 25973
Got an event from pending child 25973 (1057f)
Hit a non-gdbserver trap event.
SEL: Found 2 SIGTRAP events, selecting #1
linux_wait_1 ret = LWP 25988.25988, 1, 0
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^  ("1" is TARGET_WAITKIND_STOPPED)
<<<< exiting linux_wait_1
Writing resume reply for LWP 25988.25988:1
ptrace(regsets_fetch_inferior_registers) PID=25988: No such process
ptrace(regsets_fetch_inferior_registers) PID=25988: No such process
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Ignore the "SIGTRAP" mention in "SEL: Found 2 SIGTRAP events",
it's "two events".  And the one that was picked was a process
exit.  But the tail end of linux_wait_1 isn't expecting that
can happen.

Thanks,
Pedro Alves