[Lldb-commits] [PATCH]race condition calling PushProcessIOHandler
mg11 at csr.com
Mon Jul 28 22:30:36 PDT 2014
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
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.
Shawn Best wrote:
> 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
> 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.
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