"I think the last suggestion of moving the sync point to
Debugger::HandleProcessEvent() would defeat the purpose of the patch
since it is called from the EventHandler thread. The race condition is
the main thread returning up the call stack to start another round of
commandIO handling before PushProcessIOHandler() gets called."
I agree with this remark of Shawn's. The whole point of this issue is to
block the main thread i.e. the one which calls
IOHandlerEditline::GetLine, which is where the (lldb) prompt is printed,
i.e.
#0 lldb_private::IOHandlerEditline::GetLine
#1 0x00007ffff770aceb in lldb_private::IOHandlerEditline::Run
#2 0x00007ffff76f546a in lldb_private::Debugger::ExecuteIOHanders
#3 0x00007ffff77d78bb in
lldb_private::CommandInterpreter::RunCommandInterpreter
#4 0x00007ffff7a47955 in lldb::SBDebugger::RunCommandInterpreter
#5 0x0000000000409234 in Driver::MainLoop
#6 0x0000000000409572 in main
I spent quite a while looking at this problem and the surrounding
architecture this morning. The observed problem occurs when the
Editline's IOHandler object does not have it's m_active flag cleared
before it gets it's Run method called again (i.e. after the last user
command is processed). So we have to block this main thread until
another IOHandler can be pushed (i.e. the process one). It is the act of
pushing an IOHandler which has the effect of clearing the m_active flag
in the previous topmost handler.
Regarding Greg's comment:
"not having to have every command add a call to
process->WaitForPushedIOHandlerSync(...)."
The problem is that it is only the individual "Command" object
implementations that know whether asynchronous processing will occur,
and hence whether this "block main thread until IOHandler pushed" action
is necessary. And therefore we may need to add this kind of logic
piece-meal. It's not great, but with my limited view of lldb's
architecture, I can't currently think of a better way.
Matt
Shawn Best wrote:
Thanks for the feedback Greg. I have attached a revised patch based
on your suggestions.
- renamed m_pushed_IOHandler to m_iohandler_sync
- got rid of method ClearPushedIOHandlerSync() and make calls directly
to m_iohandler_sync.SetValue(false..)
- renamed WaitForPushedIOHandlerSync() to SyncIOHandler()
- only wait in case where m_process_input_reader != NULL
I put the calls to reset the sync flag in both PrivateEventThread
(after it has seen a public stop), and after the call to
SyncIOHandler() is completed. As far as I can tell it should work
fine, but my preference is normally to reset the flag immediately
before using it instead of relying on it set to false. Having it
internal does clean up the code though.
I think the last suggestion of moving the sync point to
Debugger::HandleProcessEvent() would defeat the purpose of the patch
since it is called from the EventHandler thread. The race condition
is the main thread returning up the call stack to start another round
of commandIO handling before PushProcessIOHandler() gets called.
On Fri, Aug 1, 2014 at 10:39 AM, Greg Clayton <[email protected]
<mailto:[email protected]>> wrote:
Comments:
Why should anyone outside of lldb_private::Process have to call
ClearPushedIOHandlerSync() manually? Can we get rid of this
function and just have Process.cpp do it at the right times by
directly calling m_pushed_IOHandler.SetValue(false, eBroadcastNever);?
If so then the following fixes apply:
1 - remove Process::ClearPushedIOHandlerSync() since it will be
done internally within process.
2 - rename m_pushed_IOHandler to m_iohandler_sync
3 - rename WaitForPushedIOHandlerSync() to SyncIOHandler
Other general fixes:
1 - the WaitForPushedIOHandlerSync should do nothing if there is
no process IOHandler (no stdio or we attached to a process. This
can be done by testing m_process_input_reader with "if
(m_process_input_reader) { ... }"
I would also like the fix this sync issue by not having to have
every command add a call to
process->WaitForPushedIOHandlerSync(...). Can't we sync this in
the Debugger::HandleProcessEvent()?
> On Jul 31, 2014, at 2:57 PM, Shawn Best <[email protected]
<mailto:[email protected]>> wrote:
>
> oops, with the attachment this time.
>
>
> On Thu, Jul 31, 2014 at 2:56 PM, Shawn Best
<[email protected] <mailto:[email protected]>> wrote:
> Thanks everyone for the feedback. I have reworked the patch to
use Predicate <bool>, it reads much cleaner now.
>
> Shawn.
>
> On Wed, Jul 30, 2014 at 6:34 PM, Greg Clayton
<[email protected] <mailto:[email protected]>> wrote:
> You will want to use a Predicate<bool> here in stead of what you
have since it is exactly what we use a predicate for. The following:
>
> + bool m_process_running_sync; // used with
WaitForProcessRunning() synchronization
> + std::condition_variable m_condition_process_running; //
used with WaitForProcessRunning() synchronization
> + std::mutex m_mutex_process_running; // used with
WaitForProcessRunning() synchronization
>
> Is exactly what the Predicate class does: protect a value with a
mutex and condition.
>
> The above code should be replaced with:
>
> Predicate<bool> m_process_running_sync;
>
> The API on Predicate should do what you want. See the header
file at "lldb/Host/Predicate.h" and also look for other places
that use this class to wait for a value to be equal to another
value, or wait for a value to not be equal to something.
>
> Let me know when you have a patch that uses Predicate and we
will look at that.
>
> Greg
>
> > On Jul 30, 2014, at 4:03 PM, Shawn Best
<[email protected] <mailto:[email protected]>> wrote:
> >
> > I have reworked the patch to use std::condition_variable.
This particular sync mechanism was new to me, I hope I used it
correctly. Is it portable across all target platforms/compilers?
I tested on linux and OSX.
> >
> > The timeout is pretty small (1ms) but seems ample based on the
measurements I made.
> >
> >
> > On Tue, Jul 29, 2014 at 9:58 PM, Matthew Gardiner
<[email protected] <mailto:[email protected]>> wrote:
> > Cool, let us know how you get on!
> > Matt
> >
> > Shawn Best wrote:
> > Thanks for the feedback guys.
> >
> > Studying the code, I had figured going with a straight int
would in practice be most efficient and not run into
multi-threaded problems, even if initially appearing a bit risky.
I will rework it to use a std::condition_variable. That will be
more robust and readable.
> >
> > Shawn.
> >
> > On 7/29/2014 10:53 AM, Zachary Turner wrote:
> > Even better would be an std::condition_variable
> >
> >
> > On Mon, Jul 28, 2014 at 10:30 PM, Matthew Gardiner
<[email protected] <mailto:[email protected]> <mailto:[email protected]
<mailto:[email protected]>>> wrote:
> >
> > Hi Shawn,
> >
> > I use 64-bit linux and I see this issue a lot. It usually
> > manifests itself as the prompt just not being printed (or
perhaps
> > it just gets overwritten) - regardless - I invoke a
command, and
> > I don't see an (lldb) prompt when I should. So I'm well
pleased
> > that you are looking at this!
> >
> > Would it not be more robust to use a semaphore than usleep to
> > synchronise the problematic threads?
> >
> > Although I've not looked too deeply into this particular
issue,
> > whenever I've seen similar races, I found that it's almost
> > impossible to pick the right value when using a sleep
command. A
> > semaphore, though, should always ensure the waiting thread
will
> > wake precisely.
> >
> > I'd be happy to help to test such a fix.
> >
> > Matt
> >
> >
> > Shawn Best wrote:
> >
> > Hi,
> >
> > I have attached a patch which addresses 3 related race
> > conditions that cause the command line (lldb) prompt
to get
> > displayed inappropriately and make it appear it is not
> > working correctly. This issue can be seen on linux and
> > FreeBSD. I can also artificailly induce the problem
on OSX.
> >
> > The issue happens when the command handler (in the main
> > thread) issues a command such as run, step or continue.
> > After the command finishes initiating its action, it
returns
> > up the call stack and goes back into the main command loop
> > waiting for user input. Simultaneously, as the inferior
> > process starts up, the MonitorChildProcess thread picks up
> > the change and posts to the PrivateEvent thread.
> > HandePrivateEvent() then calls PushProcessIOHandler()
which
> > will disable the command IO handler and give the inferior
> > control of the TTY. To observe this on OSX, put a
> >
> > usleep(100);
> >
> > immediately prior the PushProcessIOHandler() in
> > HandlePrivateEvent.
> >
> >
> > My proposed solution is that after a 'run', 'step', or
> > 'continue' command, insert a synchronization point and
wait
> > until HandlePrivateEvent knows the inferior process is
> > running and has pushed the IO handler. One context switch
> > (<100us) is usually all the time it takes on my
machine. As
> > an additional safety, I have a timeout (currently 1ms)
so it
> > will never hang the main thread.
> >
> > Any thoughts, or suggestions would be appreciated.
> >
> > Regards,
> > Shawn.
> >
> >
> > To report this email as spam click here
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> >
> >
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