[PATCH 0/5] GNU/Linux, fix attach races/problems

[Pedro Alves <palves@redhat.com>]

Investigating an "attach" bug seemingly caused by a race, related to
the fact that on Linux we have to attach to each thread individually,
I decided to write a test that stresses that aspect of attach.  The
test constantly spawns short-lived threads, and has GDB attach, debug
a little, detach, attach, rinse/repeat a few times.

That actually exposed a set of issues, both in GDB and in
glibc/libthread_db.

One is that the test defeats the current heuristics in place: we still
fail to attach to some threads sometimes, if the program spawns them
quickly enough.  This is fixed by fetching the list of LWPs to attach
to from /proc instead of relying on libthread_db for that.

Another is that some times we'd try to attach to a bogus lwp with
PID==-1, and do a dangerous waitpid, potentially eating an event by
mistake and breaking the debug session as result.

Yet another is a nasty libthread_db event reporting mechanism race
related to detaching from the inferior just while a thread is
reporting an event, resulting in a subsequent attach session tripping
on broken libthread_db events.  We work around this by no longer using
libthread_db for thread event creation/death reporting, which is good
on its own, for being much more robust and efficient.

I've filed a couple glibc bugs as result of all this:

 Bug 17705 - nptl_db: stale thread create/death events if debugger detaches
 https://sourceware.org/bugzilla/show_bug.cgi?id=17705

 Bug 17707 - nptl_db: terminated and joined threads
 https://sourceware.org/bugzilla/show_bug.cgi?id=17707

The series fixes the GDB issues and at the same time works around the
glibc issues.

Tested on x86_64 Fedora 20, native and gdbserver.

Comments?

Pedro Alves (5):
  libthread_db: debug output should go to gdb_stdlog
  Linux: on attach, attach to lwps listed under /proc/$pid/task/
  libthread_db: Skip attaching to terminated and joined threads
  Linux: Skip thread_db thread event reporting if PTRACE_EVENT_CLONE is
    supported
  Test attaching to a program that constantly spawns short-lived threads

 gdb/gdbserver/linux-low.c                          | 152 ++++++++------------
 gdb/gdbserver/linux-low.h                          |   6 -
 gdb/gdbserver/thread-db.c                          |  13 +-
 gdb/linux-nat.c                                    |  94 +++++++++++++
 gdb/linux-nat.h                                    |   4 +
 gdb/linux-thread-db.c                              |  84 ++++++++---
 gdb/nat/linux-procfs.c                             | 153 ++++++++++++++++++---
 gdb/nat/linux-procfs.h                             |  32 ++++-
 gdb/nat/linux-ptrace.c                             |  33 ++++-
 gdb/nat/linux-ptrace.h                             |   8 ++
 .../gdb.threads/attach-many-short-lived-threads.c  | 117 ++++++++++++++++
 .../attach-many-short-lived-threads.exp            | 135 ++++++++++++++++++
 gdb/testsuite/gdb.threads/fork-thread-pending.exp  |   2 +-
 .../signal-command-multiple-signals-pending.c      |  11 +-
 .../signal-command-multiple-signals-pending.exp    |   7 +
 15 files changed, 698 insertions(+), 153 deletions(-)
 create mode 100644 gdb/testsuite/gdb.threads/attach-many-short-lived-threads.c
 create mode 100644 gdb/testsuite/gdb.threads/attach-many-short-lived-threads.exp

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1.9.3