Hi Shawn,
I've *just* started looking at this. A few initial observations:
1. When I had a paused native target, when I repeatedly stepped the
prompt always appeared as expected. Good!
2. However, when I attached to a stopped target over gdb-remote, the
prompt was not there. Did you expect that? Or is that out of the scope
of your investigation. I wondered whether such a fix should work for
that too. See below:
(lldb) target create ~/my-dir/i64-hello.elf
Current executable set to '~/my-dir/i64-hello.elf' (x86_64).
(lldb) gdb-remote 7779
Process 18048 stopped
* thread #1: tid = 18048, 0x0000003675a011f0, name = 'i64-hello.elf',
stop reason = signal SIGSTOP
frame #0: 0x0000003675a011f0
-> 0x3675a011f0: movq %rsp, %rdi
0x3675a011f3: callq 0x3675a046e0
0x3675a011f8: movq %rax, %r12
0x3675a011fb: movl 0x21eb97(%rip), %eax
(I expected a prompt here - since the target is stopped)
3. Also what's the significance of the 2000 literal? Is it a timeout?
Since we expect to wake on a signal, we could make this bigger.
I'll have a better look on monday.
thanks
Matt
Shawn Best 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.
>
>
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