[lldb-dev] Synchronous execution with process plugin
Zachary Turner
zturner at google.com
Fri Mar 20 18:35:43 PDT 2015
Hmm, on Monday I'll try to see if i can figure out where to put a
breakpoint to get hit when someone removes something from the queue. Thanks
On Fri, Mar 20, 2015 at 5:52 PM <jingham at apple.com> wrote:
>
> > On Mar 20, 2015, at 5:21 PM, Zachary Turner <zturner at google.com> wrote:
> >
> > If I do that my process does stop at main, but I think I'm hit with a
> race condition. 3 times out of 5 I saw the same results as you, where it
> doesn't print the backtrace but if I do bt it works fine. The other 2
> times out of 5 It does print the backtrace, but then I'm deadlocked. I
> added a bunch of tracepoints so we can see the order of events. here's
> what it looks like when it deadlocks.
> >
> > Target::Launch about to call ProcessWindows::Launch
> > ProcessWindows::DoLaunch entering
> > ProcessWindows::DoLaunch about to call DebugLaunch
> > ProcessWindows::DoLaunch succeeded, waiting for initial stop
> > Application "\??\D:\src\llvm\tools\lldb\test\expression_command\formatters\a.out"
> found in cache
> > ProcessWindows::OnDebuggerConnected
> > ProcessWindows::OnDebugException
> > ProcessWindows got EXCEPTION_BREAKPOINT
> > ProcessWindows got initial stop, setting initial stop event
> > ProcessWindows::DoLaunch received initial stop, returning
> > ProcessWindows::DoLaunch finished launching process, exiting
> > Target::Launch returned from ProcessWindows::DoLaunch,
> eLaunchFlagStopAtEntry == false, about to call WaitForProcessToStop
> > Target::Launch, WaitForProcessToStop returned, new state = eStateStopped
> > Target::Launch about to call ProcessWindows::PrivateResume
> > ProcessWindows::DoResume resuming from active exception
> > ProcessWindows::DoResume couldn't find an active exception, setting
> state to eStateRunning
> > ProcessWindows::OnDebugException
> > ProcessWindows got EXCEPTION_BREAKPOINT
> > Target::Launch synchronous_execution is true, calling
> WaitForProcessToStop again
> >
> > The interesting stuff is the last 2 lines. It hits the breakpoint in
> main *before* calling WaitForProcessToStop, so WaitForProcessToStop never
> sees a "change".
>
> > Here's what it looks like when it works.
> >
> > Target::Launch about to call ProcessWindows::Launch
> > ProcessWindows::DoLaunch entering
> > ProcessWindows::DoLaunch about to call DebugLaunch
> > ProcessWindows::DoLaunch succeeded, waiting for initial stop
> > Application "\??\D:\src\llvm\tools\lldb\test\expression_command\formatters\a.out"
> found in cache
> > ProcessWindows::OnDebuggerConnected
> > ProcessWindows::OnDebugException
> > ProcessWindows got EXCEPTION_BREAKPOINT
> > ProcessWindows got initial stop, setting initial stop event
> > ProcessWindows::DoLaunch received initial stop, returning
> > ProcessWindows::DoLaunch finished launching process, exiting
> > ProcessWindows breakpoint handler set private state to eStateStopped
> > Target::Launch returned from ProcessWindows::DoLaunch,
> eLaunchFlagStopAtEntry == false, about to call WaitForProcessToStop
> > Target::Launch, WaitForProcessToStop returned, new state = eStateStopped
> > Target::Launch about to call ProcessWindows::PrivateResume
> > ProcessWindows::DoResume resuming from active exception
> > ProcessWindows::DoResume couldn't find an active exception, nothing to
> resume
> > ProcessWindows::OnDebugException
> > ProcessWindows got EXCEPTION_BREAKPOINT
> > Target::Launch synchronous_execution is true, calling
> WaitForProcessToStop again
> > ProcessWindows breakpoint handler set private state to eStateStopped
> > Target::Launch WaitForProcessToStop returned, yay!
> >
> >
> > The difference here is that "calling WaitForProcessToStop again" happens
> before the process plugin sets the private state. Can you try repeating
> your test multiple times and seeing if you can get the deadlock to occur?
>
> I can't get this to fail on OS X but with timing related things I'm not
> sure that's all that significant...
>
> Anyway, what you are describing doesn't make sense to me yet. After
> calling PrivateResume, we call WaitForProcessToStop, passing "wait_always"
> set to true. When wait_always is true, WaitForProcessToStop won't return
> till it receives a stop event. It isn't comparing current state against
> desired state, it is actually waiting for an event. So it should not
> matter when the breakpoint hit arrives relative to calling
> "WaitForProcessToStop" since the breakpoint hit is just going to generate a
> stop event, which will sit in the event queue till somebody fetches it.
> It's fine for that to happen before you call WaitForProcessToStop, because
> we aren't comparing current to desired state, we are waiting for that event.
>
> Maybe somebody else is grabbing the breakpoint stop event and taking it
> off of the queue? That would explain why the second WaitForProcessToStop -
> the one with wait_always set to true - is missing the event?
>
> Jim
>
>
> >
> >
> > On Fri, Mar 20, 2015 at 4:49 PM <jingham at apple.com> wrote:
> > I'm not sure it is deadlocking the debugger. lldb is just waiting for a
> stop. For instance ^C should interrupt it, or sending a signal externally
> to the process, or triggering a breakpoint or crash, etc.
> >
> > Actually, Greg must have fixed the bug I was remembering, because this
> works correctly for me with TOT lldb.
> >
> > What happens for you if your .lldbinit has:
> >
> > file a.out
> > break set -n main
> > run
> >
> > For me this stops at the breakpoint at main. We still have a little
> clean up to do here, because I don't see the stop notification in this
> case. I see:
> >
> > > lldb -S cmds.lldb
> > (lldb) command source -s 1 'cmds.lldb'
> > 4 locations added to breakpoint 1
> > (lldb)
> >
> > but if I then do "bt" I'm sitting at main:
> >
> > (lldb) bt
> > * thread #1: tid = 0x1bf8a4, function: main , stop reason = breakpoint
> 1.1
> > * frame #0: 0x0000000100018e87 Sketch`main at SKTMain.m:17
> > frame #1: 0x00007fff94f445ad libdyld.dylib`start
> > frame #2: 0x00007fff94f445ad libdyld.dylib`start
> >
> > Not sure what's up with the stutter at start either. But that's a
> different rabbit to chase...
> >
> > Jim
> >
> >
> > > On Mar 20, 2015, at 4:40 PM, Zachary Turner <zturner at google.com>
> wrote:
> > >
> > > If that's the case, then a .lldbinit file like this:
> > >
> > > file a.out
> > > run
> > >
> > > Will deadlock the debugger, because the real stop never comes?
> > >
> > > On Fri, Mar 20, 2015 at 4:35 PM <jingham at apple.com> wrote:
> > > That's the stop at entry stop. The code you quoted is in a block that
> starts with:
> > >
> > > if (launch_info.GetFlags().Test(eLaunchFlagStopAtEntry) ==
> false)
> > > {
> > >
> > > So we've stopped at the entry point, but the user didn't want to know
> about that, so we resume and wait for a "real" stop.
> > >
> > > Jim
> > >
> > >
> > > > On Mar 20, 2015, at 4:30 PM, Zachary Turner <zturner at google.com>
> wrote:
> > > >
> > > > I'm a little confused. You said that in synchronous execution,
> Launch won't return until the process has stopped. That makes sense, but
> it already checks that the process has stopped once regardless of whether
> synchronous execution is set. Then, it calls PrivateResume() (even if
> synchronous_execution is set), and then waits for the process to stop
> again? What would trigger this second stop? Target::Launch already asked
> it to resume, so now it's happily running while Target::Launch is waiting
> for it to stop a second time.
> > > >
> > > > On Fri, Mar 20, 2015 at 4:23 PM <jingham at apple.com> wrote:
> > > > In synchronous execution, the "Launch" command won't return till the
> process has stopped. The point of synchronous execution is that you can do:
> > > >
> > > > break set -n foo
> > > > run
> > > > bt
> > > >
> > > > So "run" can't return till the breakpoint has been hit. That is why
> it waits for the process to stop. I'm not quite sure why this is done in
> Target::Launch, in other cases (e.g. in for "step" and "continue" the
> command object is the one that takes care of waiting for the stop. Launch
> is a little funny however, because it can't use the normal process wait
> mechanism to do its job since the real process isn't alive when it has to
> start waiting...
> > > >
> > > > I think the reason you are hanging here is that the code that reads
> in all the init statements runs an event loop temporarily while it is
> reading them in, and the kills that and hands off the the real command
> execution loop, and this continuation gets lost in the handoff. I thought
> Greg had already fixed that, but maybe it's still sitting in his queue.
> > > >
> > > > Jim
> > > >
> > > >
> > > >
> > > >
> > > > > On Mar 20, 2015, at 3:57 PM, Zachary Turner <zturner at google.com>
> wrote:
> > > > >
> > > > > I ran into an issue earlier where I tried to make a .lldbinit file
> with some lines like this:
> > > > >
> > > > > file a.out
> > > > > run
> > > > >
> > > > >
> > > > > When this happens the process runs, the breakpoint gets hit and I
> see the source listing, it returns to the lldb prompt, but then I can't
> type anything. It appears LLDB is deadlocked inside of Target::Launch() at
> the following location:
> > > > >
> > > > > if (!synchronous_execution)
> > > > > m_process_sp->RestoreProcessEvents ();
> > > > >
> > > > > error = m_process_sp->PrivateResume();
> > > > >
> > > > > if (error.Success())
> > > > > {
> > > > > // there is a race condition where this thread
> will return up the call stack to the main command
> > > > > // handler and show an (lldb) prompt before
> HandlePrivateEvent (from PrivateStateThread) has
> > > > > // a chance to call PushProcessIOHandler()
> > > > > m_process_sp->SyncIOHandler(2000);
> > > > >
> > > > > if (synchronous_execution)
> > > > > {
> > > > >
> > > > > state = m_process_sp->WaitForProcessToStop (NULL, NULL, true,
> hijack_listener_sp.get(), stream);
> > > > > const bool must_be_alive = false; //
> eStateExited is ok, so this must be false
> > > > > if (!StateIsStoppedState(state,
> must_be_alive))
> > > > > {
> > > > > error.SetErrorStringWithFormat("process
> isn't stopped: %s", StateAsCString(state));
> > > > > }
> > > > > }
> > > > > }
> > > > >
> > > > > Normally when I'm using LLDB and entering the commands myself,
> this synchronous_execution value is not set, and everything works as
> expected. How is this supposed to work? What does my plugin need to do
> differently in order to handle this case? The process has already stopped
> once and resumed, so I'm not sure why it would need to stop again? I see
> that it's not restoring process events in the case of synchronous
> execution, so maybe it should have never resumed in the first place?
> _______________________________________________
> > > > > lldb-dev mailing list
> > > > > lldb-dev at cs.uiuc.edu
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> > > >
> > >
> >
>
>
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