[Lldb-commits] [PATCH]race condition calling PushProcessIOHandler
Shawn Best
sbest at blueshiftinc.com
Thu Jul 31 14:56:51 PDT 2014
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 <gclayton at apple.com> 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 <sbest at blueshiftinc.com> 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 <mg11 at csr.com> 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 <mg11 at csr.com
> <mailto:mg11 at csr.com>> 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.
> >
> >
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