[Lldb-commits] [PATCH]race condition calling PushProcessIOHandler

Matthew Gardiner mg11 at csr.com
Tue Jul 29 21:58:41 PDT 2014

Cool, let us know how you get on!

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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