From 30e0fca6c1d7d26f3f2daa4dd2b12c51dadc778a Mon Sep 17 00:00:00 2001 From: Andrea Arcangeli Date: Sun, 30 Oct 2005 15:02:38 -0800 Subject: [PATCH] [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:{try_to_wake_up+893} {finish_stop+87} {get_signal_to_deliver+1359} {do_signal+157} {ptrace_check_attach+222} {sys_ptrace+2293} {default_wake_function+0} {sys_ioctl+73} {sysret_signal+28} {ptregscall_common+103} test D ffff810011362100 0 14309 1 14305 14312 (NOTLB) ffff810053c81cf8 0000000000000082 0000000000000286 0000000000000001 0000000000000195 ffff810011362340 ffff810011362100 ffff81002e338040 ffff810001e0ca80 0000000000000001 Call Trace:{try_to_wake_up+893} {wait_for_completion+173} {default_wake_function+0} {exit_mm+149} {do_exit+479} {do_group_exit+252} {get_signal_to_deliver+1451} {do_signal+157} {ptrace_check_attach+222} {specific_send_sig_info+2 {force_sig_info+186} {do_int3+112} {retint_signal+61} test D ffff81002e338040 0 14311 1 14716 14305 (NOTLB) ffff81005ca8dcf8 0000000000000082 0000000000000286 0000000000000001 0000000000000120 ffff81002e338280 ffff81002e338040 ffff8100481cb740 ffff810001e0ca80 0000000000000001 Call Trace:{try_to_wake_up+893} {wait_for_completion+173} {default_wake_function+0} {exit_mm+149} {do_exit+479} {__dequeue_signal+558} {do_group_exit+252} {get_signal_to_deliver+1451} {do_signal+157} {ptrace_check_attach+222} {specific_send_sig_info+208} {force_sig_info+186} {do_int3+112} {retint_signal+61} test D ffff810017ca4180 0 14312 1 14309 13882 (NOTLB) ffff81005d15fcb8 0000000000000082 ffff81005d15fc58 ffffffff80130816 0000000000000897 ffff810017ca43c0 ffff810017ca4180 ffff81003e8118c0 0000000000000082 ffffffff801317ed Call Trace:{activate_task+150} {try_to_wake_up+893} {wait_for_completion+173} {default_wake_function+0} {do_coredump+819} {thread_return+82} {get_signal_to_deliver+1444} {do_signal+157} {ptrace_check_attach+222} {specific_send_sig_info+2 {_spin_unlock_irqrestore+5} {force_sig_info+186} {do_general_protection+159} {retint_signal+61} Signed-off-by: Andrea Arcangeli Cc: Roland McGrath Cc: Ingo Molnar Cc: Linus Torvalds Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/ptrace.c | 7 +++++-- kernel/signal.c | 6 +++--- 2 files changed, 8 insertions(+), 5 deletions(-) diff --git a/kernel/ptrace.c b/kernel/ptrace.c index 019e04ec065a..863eee8bff47 100644 --- a/kernel/ptrace.c +++ b/kernel/ptrace.c @@ -56,6 +56,10 @@ void ptrace_untrace(task_t *child) signal_wake_up(child, 1); } } + if (child->signal->flags & SIGNAL_GROUP_EXIT) { + sigaddset(&child->pending.signal, SIGKILL); + signal_wake_up(child, 1); + } spin_unlock(&child->sighand->siglock); } @@ -77,8 +81,7 @@ void __ptrace_unlink(task_t *child) SET_LINKS(child); } - if (child->state == TASK_TRACED) - ptrace_untrace(child); + ptrace_untrace(child); } /* diff --git a/kernel/signal.c b/kernel/signal.c index 1d905ec74bde..9d1512dcf176 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -1108,8 +1108,8 @@ void zap_other_threads(struct task_struct *p) if (t != p->group_leader) t->exit_signal = -1; + /* SIGKILL will be handled before any pending SIGSTOP */ sigaddset(&t->pending.signal, SIGKILL); - rm_from_queue(SIG_KERNEL_STOP_MASK, &t->pending); signal_wake_up(t, 1); } } @@ -1879,9 +1879,9 @@ relock: /* Let the debugger run. */ ptrace_stop(signr, signr, info); - /* We're back. Did the debugger cancel the sig? */ + /* We're back. Did the debugger cancel the sig or group_exit? */ signr = current->exit_code; - if (signr == 0) + if (signr == 0 || current->signal->flags & SIGNAL_GROUP_EXIT) continue; current->exit_code = 0; -- 2.30.2