[Bug gdb/12702] gdb can hang waiting for thread group leader

["cvs-commit at gcc dot gnu.org" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=12702

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https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=fa96cb382c12b099675c5cc238aaa7352a3fd3d7

commit fa96cb382c12b099675c5cc238aaa7352a3fd3d7
Author: Pedro Alves <palves@redhat.com>
Date:   Thu Feb 27 14:30:08 2014 +0000

    Teach GDBserver's Linux backend about no unwaited-for children
(TARGET_WAITKIND_NO_RESUMED).

    GDBserver currently hangs forever in waitpid if the leader thread
    exits before other threads, or if all resumed threads exit - e.g.,
    next over a thread exit with sched-locking on.  This is exposed by
    leader-exit.exp.  leader-exit.exp is part of a series of tests for a
    set of related problems.  See
    <http://www.sourceware.org/ml/gdb-patches/2011-10/msg00704.html>:

     "
     To recap, on the Linux kernel, ptrace/waitpid don't allow reaping the
     leader thread until all other threads in the group are reaped.  When
     the leader exits, it goes zombie, but waitpid will not return an exit
     status until the other threads are gone.  This is presently exercised
     by the gdb.threads/leader-exit.exp test.  The fix for that test, in
     linux-nat.c:wait_lwp, handles the case where we see the leader gone
     when we're stopping all threads to report an event to some other
     thread to the core.

     (...)

     The latter bit about not blocking if there no resumed threads in the
     process also applies to some other thread exiting, not just the main
     thread.  E.g., this test starts a thread, and runs to a breakpoint in
     that thread:

     ...
     (gdb) c
     Continuing.
     [New Thread 0x7ffff75a4700 (LWP 23397)]
     [Switching to Thread 0x7ffff75a4700 (LWP 23397)]

     Breakpoint 2, thread_a (arg=0x0) at
../../../src/gdb/testsuite/gdb.threads/no-unwaited-for-left.c:28
     28        return 0; /* break-here */
     (gdb) info threads
     * 2 Thread 0x7ffff75a4700 (LWP 23397)  thread_a (arg=0x0) at
../../../src/gdb/testsuite/gdb.threads/no-unwaited-for-left.c:28
       1 Thread 0x7ffff7fcb720 (LWP 23391)  0x00007ffff7bc606d in pthread_join
(threadid=140737343276800, thread_return=0x0) at pthread_join.c:89

     The thread will exit as soon as we resume it.  But if we only resume
     that thread, leaving the rest of the threads stopped:

     (gdb) set scheduler-locking on
     (gdb) c
     Continuing.
     ^C^C^C^C^C^C^C^C
     "

    This patch fixes the issues by implementing TARGET_WAITKIND_NO_RESUMED
    on GDBserver, similarly to what the patch above did for native
    Linux GDB.

    gdb.threads/leader-exit.exp now passes.

    gdb.threads/no-unwaited-for-left.exp now at least errors out instead
    of hanging:

     continue
     Continuing.
     warning: Remote failure reply: E.No unwaited-for children left.

     [Thread 15454] #1 stopped.
     0x00000034cf408e60 in pthread_join (threadid=140737353922368,
thread_return=0x0) at pthread_join.c:93
     93          lll_wait_tid (pd->tid);
     (gdb) FAIL: gdb.threads/no-unwaited-for-left.exp: continue stops when the
main thread exits

    The gdb.threads/non-ldr-exc-*.exp tests are skipped because GDBserver
    unfortunately doesn't support fork/exec yet, but I'm confident this
    fixes the related issues.

    I'm leaving modeling TARGET_WAITKIND_NO_RESUMED in the RSP for a
    separate pass.

    (BTW, in case of error in response to a vCont, it would be better for
    GDB to query the target for the current thread, or re-select one,
    instead of assuming current inferior_ptid is still the selected
    thread.)

    This implementation is a little different from GDB's, because I'm
    avoiding bringing in more of this broken use of waitpid(PID) into
    GDBserver.  Specifically, this avoids waitpid(PID) when stopping all
    threads.  There's really no need for wait_for_sigstop to wait for each
    LWP in turn.  Instead, with some refactoring, we make it reuse
    linux_wait_for_event.

    gdb/gdbserver/
    2014-02-27  Pedro Alves  <palves@redhat.com>

        PR 12702
        * inferiors.h (A_I_NEXT, ALL_INFERIORS_TYPE, ALL_PROCESSES): New
        macros.
        * linux-low.c (delete_lwp, handle_extended_wait): Add debug
        output.
        (last_thread_of_process_p): Take a PID argument instead of a
        thread pointer.
        (linux_wait_for_lwp): Delete.
        (num_lwps, check_zombie_leaders, not_stopped_callback): New
        functions.
        (linux_low_filter_event): New function, party factored out from
        linux_wait_for_event.
        (linux_wait_for_event): Rename to ...
        (linux_wait_for_event_filtered): ... this.  Add new filter ptid
        argument.  Partly rewrite.  Always use waitpid(-1, WNOHANG) and
        sigsuspend.  Check for zombie leaders.
        (linux_wait_for_event): Reimplement as wrapper around
        linux_wait_for_event_filtered.
        (linux_wait_1): Handle TARGET_WAITKIND_NO_RESUMED.  Assume that if
        a normal or signal exit is seen, it's the whole process exiting.
        (wait_for_sigstop): No longer a for_each_inferior callback.
        Rewrite on top of linux_wait_for_event_filtered.
        (stop_all_lwps): Call wait_for_sigstop directly.
        * server.c (resume, handle_target_event): Handle
        TARGET_WAITKIND_NO_RESUMED.

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=d632a0971cf9c8044e2b59da45ba6bbc27714c84

commit d632a0971cf9c8044e2b59da45ba6bbc27714c84
Author: Pedro Alves <palves@redhat.com>
Date:   Thu Feb 27 14:30:08 2014 +0000

    Move linux-nat.c:status_to_str to nat/linux-waitpid.c.

    So that gdbserver's Linux backend can use it too.

    gdb/
    2014-02-27  Pedro Alves  <palves@redhat.com>

        PR 12702
        * linux-nat.c (status_to_str): Moved to nat/linux-waitpid.c.
        * nat/linux-waitpid.c: Include string.h.
        (status_to_str): Moved here and made extern.
        * nat/linux-waitpid.h (status_to_str): New declaration.

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=2ebd5a35208e40dab7df46c7d6d58f4d614c6e21

commit 2ebd5a35208e40dab7df46c7d6d58f4d614c6e21
Author: Hui Zhu <hui@codesourcery.com>
Date:   Thu Feb 27 14:30:07 2014 +0000

    Move ptid_match to common/ptid.c.

    So that gdbserver can use it too.

    gdb/
    2014-02-27  Hui Zhu  <hui@codesourcery.com>

        PR 12702
        * infrun.c (ptid_match): Move ...
        * common/ptid.c (ptid_match): ... here.
        * inferior.h (ptid_match): Move ...
        * common/ptid.h (ptid_match): ... here.

-----------------------------------------------------------------------

Summary of changes:
 gdb/ChangeLog             |   16 +
 gdb/common/ptid.c         |   14 +
 gdb/common/ptid.h         |   10 +
 gdb/gdbserver/ChangeLog   |   28 ++
 gdb/gdbserver/inferiors.h |   22 ++
 gdb/gdbserver/linux-low.c |  852 ++++++++++++++++++++++++++-------------------
 gdb/gdbserver/server.c    |   18 +-
 gdb/inferior.h            |   10 -
 gdb/infrun.c              |   14 -
 gdb/linux-nat.c           |   25 --
 gdb/nat/linux-waitpid.c   |   28 ++
 gdb/nat/linux-waitpid.h   |    4 +
 12 files changed, 640 insertions(+), 401 deletions(-)

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