[lldb-dev] Synchronous execution with process plugin
Zachary Turner
zturner at google.com
Fri Mar 20 17:21:54 PDT 2015
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?
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
> > > > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
> > >
> >
>
>
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