[lldb-dev] gdb-remote lldb prompt race condition
Todd Fiala
tfiala at google.com
Thu Aug 21 08:25:18 PDT 2014
> I'm wondering whether whether we need some brave soul, to redesign the
current lldb IO handling mechanisms.
I'm not sure about the "brave soul" part, but my team may look to take this
on. We're getting hammered by it in several different contexts (i.e. the
idea that races here seem to require several one-off fixes to get the
handling right). When we get a few cycles on it we may try to hash out
something coherent and propose it here. (If somebody beats us to it, have
at it).
I'm blocked by I think a related issue here, so I may get to this sooner
than later.
-Todd
On Thu, Aug 21, 2014 at 12:27 AM, Matthew Gardiner <mg11 at csr.com> wrote:
> Hi Greg,
>
> If that is the case, then the main thread (i.e. the one running
> Debugger::ExecuteIOHandler) needs to be blocked until the event arrives.
>
> Why?
>
> Because with the existing design once the user has entered their
> "gdb-remote" command, and completed the connect call, the main thread goes
> back to the IOHandlerEditline::Run() loop, sees that IsActive() is true,
> prints the next prompt, etc.. When I debugged this I didn't see any call to
> Deactivate or SetIsDone, which would have made IsActive return false. (And
> then the async DefaultEventHandler awakes and it's output "Process 1
> stopped" splats over the prompt).
>
> If the code is changed so that the edit line IOHandler's IsActive returns
> false, while an asynchronous event is happening, then I think that the main
> thread would spin, since the reader_sp->Run() function below:
>
> void
> Debugger::ExecuteIOHanders()
> {
> while (1)
> {
> IOHandlerSP reader_sp(m_input_reader_stack.Top());
> if (!reader_sp)
> break;
>
> reader_sp->Activate();
> reader_sp->Run();
> reader_sp->Deactivate();
>
> would immediately return. That's why I'm thinking the main thread probably
> should block until the last issued command has completed.
>
> Out of interest, I did research your "If someone is grabbing the event
> manually by hijacking events" point. But when stopped state is detected
> (i.e. the reply to ?) in GDBRemote and Broadcaster::PrivateBroadcastEvent
> is called, there is no hijacking_listener. Indeed the execution path is
> that as indicated by the -->
>
> if (hijacking_listener)
> {
> if (unique && hijacking_listener->PeekAtNextEventForBroadcasterWithType
> (this, event_type))
> return;
> hijacking_listener->AddEvent (event_sp);
> }
> else
> {
> collection::iterator pos, end = m_listeners.end();
> // Iterate through all listener/mask pairs
> for (pos = m_listeners.begin(); pos != end; ++pos)
> {
> // If the listener's mask matches any bits that we just set,
> then
> // put the new event on its event queue.
> if (event_type & pos->second)
> {
> if (unique && pos->first->PeekAtNextEventForBroadcasterWithType
> (this, event_type))
> continue;
> ----> pos->first->AddEvent (event_sp);
>
> So my contention is that in the case of gdb-connect the initial stop state
> should either be delivered/printed sychronously in the
> Process::ConnectRemote (i.e. in the mainthread context) or that the main
> thread should block until either the event arrives, or for some other
> reason the command last issued by the user is deemed to be "complete".
>
> thanks
> Matt
>
>
>
> Greg Clayton wrote:
>
>> The event should get delivered to the Debugger thread that is waiting for
>> events and it should coordinate with the top io handler when printing it.
>> If someone is grabbing the event manually by hijacking events, then we need
>> to fix that code to send the event on to the unhijacked main event loop.
>>
>>
>> On Aug 20, 2014, at 1:42 AM, Matthew Gardiner <mg11 at csr.com> wrote:
>>>
>>> Hi folks
>>>
>>> I have been seeing another issue with the display of the lldb prompt.
>>> This time it's when I do "target create elf-file", then "gdb-remote
>>> port-number". After the "gdb-remote" command I see the fact that my process
>>> is stopped, e.g.
>>>
>>> Process 1 stopped
>>>
>>> on the screen. But no (lldb) prompt.
>>>
>>> Some investigation revealed that what's *probably* happening is the main
>>> thread after processing the "gdb-remote" returns to it's IOHandler, which
>>> then prints (lldb). However, the inferior's state changes seem to delivered
>>> to stdout via a different thread, basically one which sits in
>>> Debugger::DefaultEventHandler. This subsequent output then, I think,
>>> overwrites the previous (lldb) prompt.
>>>
>>> Now in my (and presumably other people's) situation, this issue is
>>> compounded by the speed of the TCP/IP connection to the gdbserver stub, the
>>> "poll the hardware" nature of my stub, and the fact the hardware is
>>> actually simulated - yes over a TCP/IP socket.
>>>
>>> FWIW, I resolved this by a horrible (POSIX only) hack, of sleeping for
>>> 300ms at the bottom of the CommandInterpreter::HandleCommand function.
>>>
>>> @@ -9,6 +9,8 @@
>>>
>>> #include "lldb/lldb-python.h"
>>>
>>> +#include <poll.h> // MG for prompt bugs
>>> +
>>> #include <string>
>>> #include <vector>
>>> #include <stdlib.h>
>>> @@ -1916,6 +1918,9 @@
>>> if (log)
>>> log->Printf ("HandleCommand, command %s", (result.Succeeded() ?
>>> "succeeded" : "did not succeed"));
>>>
>>> + // MG wait for remote connects etc. to complete
>>> + poll(0,0,300);
>>> +
>>> return result.Succeeded();
>>> }
>>>
>>>
>>> But this is horrid. Given this, and other prompt issues, I'm wondering
>>> whether whether we need some brave soul, to redesign the current lldb IO
>>> handling mechanisms.
>>>
>>> Matt
>>>
>>>
>>>
>>>
>>>
>>>
>>>
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--
Todd Fiala | Software Engineer | tfiala at google.com | 650-943-3180
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