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
> > <https://www.mailcontrol.com/sr/MZbqvYs5QwJvpeaetUwhCQ==>.
> >
> >
> >
> > _______________________________________________
> > lldb-commits mailing list
> > [email protected] <mailto:[email protected]>
<mailto:[email protected]
<mailto:[email protected]>>
> >
> > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-commits
> >
> >
> >
> >
> > Member of the CSR plc group of companies. CSR plc
registered in
> > England and Wales, registered number 4187346, registered
office
> > Churchill House, Cambridge Business Park, Cowley Road,
Cambridge,
> > CB4 0WZ, United Kingdom
> > More information can be found at www.csr.com
<http://www.csr.com>
> > <http://www.csr.com>. Keep up to date with CSR on our
technical
> > blog, www.csr.com/blog <http://www.csr.com/blog>
<http://www.csr.com/blog>, CSR people
> > blog, www.csr.com/people <http://www.csr.com/people>
<http://www.csr.com/people>, YouTube,
> > www.youtube.com/user/CSRplc
<http://www.youtube.com/user/CSRplc>
<http://www.youtube.com/user/CSRplc>,
> > Facebook, www.facebook.com/pages/CSR/191038434253534
<http://www.facebook.com/pages/CSR/191038434253534>
> > <http://www.facebook.com/pages/CSR/191038434253534>, or
follow us
> > on Twitter at www.twitter.com/CSR_plc
<http://www.twitter.com/CSR_plc>
> > <http://www.twitter.com/CSR_plc>.
> >
> > New for 2014, you can now access the wide range of
products
> > powered by aptX at www.aptx.com <http://www.aptx.com>
<http://www.aptx.com>.
> > _______________________________________________
> > lldb-commits mailing list
> > [email protected] <mailto:[email protected]>
<mailto:[email protected]
<mailto:[email protected]>>
> > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-commits
> >
> >
> >
> >
> >
> >
<sbest_iohandler_race_rev_04.diff>_______________________________________________
> > lldb-commits mailing list
> > [email protected] <mailto:[email protected]>
> > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-commits
>
>
>
> <sbest_iohandler_race_rev_05.diff>
