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[PATCH 0/5] GNU/Linux, fix attach races/problems
- From: Pedro Alves <palves at redhat dot com>
- To: gdb-patches at sourceware dot org
- Date: Tue, 16 Dec 2014 16:53:49 +0000
- Subject: [PATCH 0/5] GNU/Linux, fix attach races/problems
- Authentication-results: sourceware.org; auth=none
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
--
1.9.3