linux/kernel
Andrea Arcangeli 30e0fca6c1 [PATCH] ptrace/coredump/exit_group deadlock
I could seldom reproduce a deadlock with a task not killable in T state
(TASK_STOPPED, not TASK_TRACED) by attaching a NPTL threaded program to
gdb, by segfaulting the task and triggering a core dump while some other
task is executing exit_group and while one task is in ptrace_attached
TASK_STOPPED state (not TASK_TRACED yet).  This originated from a gdb
bugreport (the fact gdb was segfaulting the task wasn't a kernel bug), but
I just incidentally noticed the gdb bug triggered a real kernel bug as
well.

Most threads hangs in exit_mm because the core_dumping is still going, the
core dumping hangs because the stopped task doesn't exit, the stopped task
can't wakeup because it has SIGNAL_GROUP_EXIT set, hence the deadlock.

To me it seems that the problem is that the force_sig_specific(SIGKILL) in
zap_threads is a noop if the task has PF_PTRACED set (like in this case
because gdb is attached).  The __ptrace_unlink does nothing because the
signal->flags is set to SIGNAL_GROUP_EXIT|SIGNAL_STOP_DEQUEUED (verified).

The above info also shows that the stopped task hit a race and got the stop
signal (presumably by the ptrace_attach, only the attach, state is still
TASK_STOPPED and gdb hangs waiting the core before it can set it to
TASK_TRACED) after one of the thread invoked the core dump (it's the core
dump that sets signal->flags to SIGNAL_GROUP_EXIT).

So beside the fact nobody would wakeup the task in __ptrace_unlink (the
state is _not_ TASK_TRACED), there's a secondary problem in the signal
handling code, where a task should ignore the ptrace-sigstops as long as
SIGNAL_GROUP_EXIT is set (or the wakeup in __ptrace_unlink path wouldn't be
enough).

So I attempted to make this patch that seems to fix the problem.  There
were various ways to fix it, perhaps you prefer a different one, I just
opted to the one that looked safer to me.

I also removed the clearing of the stopped bits from the zap_other_threads
(zap_other_threads was safe unlike zap_threads).  I don't like useless
code, this whole NPTL signal/ptrace thing is already unreadable enough and
full of corner cases without confusing useless code into it to make it even
less readable.  And if this code is really needed, then you may want to
explain why it's not being done in the other paths that sets
SIGNAL_GROUP_EXIT at least.

Even after this patch I still wonder who serializes the read of
p->ptrace in zap_threads.

Patch is called ptrace-core_dump-exit_group-deadlock-1.

This was the trace I've got:

test          T ffff81003e8118c0     0 14305      1         14311 14309 (NOTLB)
ffff810058ccdde8 0000000000000082 000001f4000037e1 ffff810000000013
       00000000000000f8 ffff81003e811b00 ffff81003e8118c0 ffff810011362100
       0000000000000012 ffff810017ca4180
Call Trace:<ffffffff801317ed>{try_to_wake_up+893} <ffffffff80141677>{finish_stop+87}
       <ffffffff8014367f>{get_signal_to_deliver+1359} <ffffffff8010d3ad>{do_signal+157}
       <ffffffff8013deee>{ptrace_check_attach+222} <ffffffff80111575>{sys_ptrace+2293}
       <ffffffff80131810>{default_wake_function+0} <ffffffff80196399>{sys_ioctl+73}
       <ffffffff8010dd27>{sysret_signal+28} <ffffffff8010e00f>{ptregscall_common+103}

test          D ffff810011362100     0 14309      1         14305 14312 (NOTLB)
ffff810053c81cf8 0000000000000082 0000000000000286 0000000000000001
       0000000000000195 ffff810011362340 ffff810011362100 ffff81002e338040
       ffff810001e0ca80 0000000000000001
Call Trace:<ffffffff801317ed>{try_to_wake_up+893} <ffffffff8044677d>{wait_for_completion+173}
       <ffffffff80131810>{default_wake_function+0} <ffffffff80137435>{exit_mm+149}
       <ffffffff801381af>{do_exit+479} <ffffffff80138d0c>{do_group_exit+252}
       <ffffffff801436db>{get_signal_to_deliver+1451} <ffffffff8010d3ad>{do_signal+157}
       <ffffffff8013deee>{ptrace_check_attach+222} <ffffffff80140850>{specific_send_sig_info+2

       <ffffffff8014208a>{force_sig_info+186} <ffffffff804479a0>{do_int3+112}
       <ffffffff8010e308>{retint_signal+61}
test          D ffff81002e338040     0 14311      1         14716 14305 (NOTLB)
ffff81005ca8dcf8 0000000000000082 0000000000000286 0000000000000001
       0000000000000120 ffff81002e338280 ffff81002e338040 ffff8100481cb740
       ffff810001e0ca80 0000000000000001
Call Trace:<ffffffff801317ed>{try_to_wake_up+893} <ffffffff8044677d>{wait_for_completion+173}
       <ffffffff80131810>{default_wake_function+0} <ffffffff80137435>{exit_mm+149}
       <ffffffff801381af>{do_exit+479} <ffffffff80142d0e>{__dequeue_signal+558}
       <ffffffff80138d0c>{do_group_exit+252} <ffffffff801436db>{get_signal_to_deliver+1451}
       <ffffffff8010d3ad>{do_signal+157} <ffffffff8013deee>{ptrace_check_attach+222}
       <ffffffff80140850>{specific_send_sig_info+208} <ffffffff8014208a>{force_sig_info+186}
       <ffffffff804479a0>{do_int3+112} <ffffffff8010e308>{retint_signal+61}

test          D ffff810017ca4180     0 14312      1         14309 13882 (NOTLB)
ffff81005d15fcb8 0000000000000082 ffff81005d15fc58 ffffffff80130816
       0000000000000897 ffff810017ca43c0 ffff810017ca4180 ffff81003e8118c0
       0000000000000082 ffffffff801317ed
Call Trace:<ffffffff80130816>{activate_task+150} <ffffffff801317ed>{try_to_wake_up+893}
       <ffffffff8044677d>{wait_for_completion+173} <ffffffff80131810>{default_wake_function+0}
       <ffffffff8018cdc3>{do_coredump+819} <ffffffff80445f52>{thread_return+82}
       <ffffffff801436d4>{get_signal_to_deliver+1444} <ffffffff8010d3ad>{do_signal+157}
       <ffffffff8013deee>{ptrace_check_attach+222} <ffffffff80140850>{specific_send_sig_info+2

       <ffffffff804472e5>{_spin_unlock_irqrestore+5} <ffffffff8014208a>{force_sig_info+186}
       <ffffffff804476ff>{do_general_protection+159} <ffffffff8010e308>{retint_signal+61}

Signed-off-by: Andrea Arcangeli <andrea@suse.de>
Cc: Roland McGrath <roland@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2005-10-30 17:37:22 -08:00
..
irq [PATCH] CHECK_IRQ_PER_CPU() to avoid dead code in __do_IRQ() 2005-09-07 16:57:29 -07:00
power [PATCH] introduce .valid callback for pm_ops 2005-10-30 17:37:15 -08:00
acct.c [PATCH] mm: rss = file_rss + anon_rss 2005-10-29 21:40:38 -07:00
audit.c [PATCH] gfp_t: kernel/* 2005-10-28 08:16:49 -07:00
auditsc.c [PATCH] gfp_t: kernel/* 2005-10-28 08:16:49 -07:00
capability.c [PATCH] kernel/capability.c: add kerneldoc 2005-07-27 16:26:06 -07:00
compat.c [PATCH] kernel: fix-up schedule_timeout() usage 2005-09-10 10:06:37 -07:00
configs.c Linux-2.6.12-rc2 2005-04-16 15:20:36 -07:00
cpu.c [PATCH] create and destroy cpufreq sysfs entries based on cpu notifiers 2005-10-30 17:37:14 -08:00
cpuset.c [PATCH] cpusets: automatic numa mempolicy rebinding 2005-10-30 17:37:22 -08:00
crash_dump.c [PATCH] kernel/crash_dump.c: add kerneldoc 2005-07-27 16:26:06 -07:00
dma.c Linux-2.6.12-rc2 2005-04-16 15:20:36 -07:00
exec_domain.c Linux-2.6.12-rc2 2005-04-16 15:20:36 -07:00
exit.c [PATCH] mm: update_hiwaters just in time 2005-10-29 21:40:39 -07:00
extable.c Linux-2.6.12-rc2 2005-04-16 15:20:36 -07:00
fork.c [PATCH] mm: ptd_alloc take ptlock 2005-10-29 21:40:40 -07:00
futex.c [PATCH] mm: follow_page with inner ptlock 2005-10-29 21:40:41 -07:00
intermodule.c [PATCH] introduce and use kzalloc 2005-09-07 16:57:45 -07:00
itimer.c [PATCH] itimer fixes 2005-07-27 16:25:51 -07:00
kallsyms.c [PATCH] ppc32: platform-specific functions missing from kallsyms. 2005-05-05 16:36:31 -07:00
Kconfig.hz [PATCH] i386: Selectable Frequency of the Timer Interrupt 2005-06-23 09:45:10 -07:00
Kconfig.preempt [PATCH] sched: voluntary kernel preemption 2005-06-25 16:24:45 -07:00
kexec.c [PATCH] mm: split page table lock 2005-10-29 21:40:42 -07:00
kfifo.c [PATCH] gfp flags annotations - part 1 2005-10-08 15:00:57 -07:00
kmod.c [PATCH] Keys: Pass session keyring to call_usermodehelper() 2005-06-24 00:05:18 -07:00
kprobes.c [PATCH] kprobes: fix bug when probed on task and isr functions 2005-09-07 16:58:01 -07:00
ksysfs.c [PATCH] Kdump: Export crash notes section address through sysfs 2005-06-25 16:24:51 -07:00
kthread.c [PATCH] Add kthread_stop_sem() 2005-10-30 17:37:17 -08:00
Makefile [PATCH] Remove redundant configs.o 2005-10-30 17:37:10 -08:00
module.c [PATCH] use add_taint() for setting tainted bit flags 2005-09-13 08:22:29 -07:00
panic.c [PATCH] Call emergency_reboot from panic 2005-07-26 14:35:43 -07:00
params.c [PATCH] Ignore trailing whitespace on kernel parameters correctly 2005-09-28 07:46:41 -07:00
pid.c Linux-2.6.12-rc2 2005-04-16 15:20:36 -07:00
posix-cpu-timers.c [PATCH] Yet more posix-cpu-timer fixes 2005-10-27 09:08:43 -07:00
posix-timers.c [PATCH] posix-timers: use schedule_timeout() in common_nsleep() 2005-10-30 17:37:20 -08:00
printk.c [PATCH] Add printk_clock() 2005-09-21 10:11:54 -07:00
profile.c [PATCH] mostly_read data section 2005-07-07 18:23:46 -07:00
ptrace.c [PATCH] ptrace/coredump/exit_group deadlock 2005-10-30 17:37:22 -08:00
rcupdate.c [PATCH] rcu: keep rcu callback event counter 2005-10-17 15:27:58 -07:00
resource.c [PATCH] introduce and use kzalloc 2005-09-07 16:57:45 -07:00
sched.c [PATCH] mm: update_hiwaters just in time 2005-10-29 21:40:39 -07:00
seccomp.c Linux-2.6.12-rc2 2005-04-16 15:20:36 -07:00
signal.c [PATCH] ptrace/coredump/exit_group deadlock 2005-10-30 17:37:22 -08:00
softirq.c [PATCH] x86-64: Some cleanup and optimization to the processor data area. 2005-09-12 10:49:58 -07:00
softlockup.c [PATCH] detect soft lockups 2005-09-07 16:57:17 -07:00
spinlock.c [PATCH] spinlock consolidation 2005-09-10 10:06:21 -07:00
stop_machine.c [PATCH] smp_processor_id() cleanup 2005-06-21 18:46:13 -07:00
sys_ni.c [PATCH] remove sys_set_zone_reclaim() 2005-08-01 10:03:56 -07:00
sys.c [PATCH] reboot: comment and factor the main reboot functions 2005-09-22 22:17:33 -07:00
sysctl.c [NET]: Fix sparse warnings 2005-08-29 16:01:32 -07:00
time.c [PATCH] NTP shift_right cleanup 2005-10-30 17:37:18 -08:00
timer.c [PATCH] remove timer debug field 2005-10-30 17:37:18 -08:00
uid16.c Linux-2.6.12-rc2 2005-04-16 15:20:36 -07:00
user.c [PATCH] inotify 2005-07-12 20:38:38 -07:00
wait.c Linux-2.6.12-rc2 2005-04-16 15:20:36 -07:00
workqueue.c [PATCH] Use alloc_percpu to allocate workqueues locally 2005-10-30 17:37:18 -08:00