Hi Shawn,
Yes, the condition variable paradigm can initially be a little tricky to
grasp. Have a look at something like
http://pages.cs.wisc.edu/~remzi/OSTEP/threads-cv.pdf, first. I think the
text on page on page 14 sums up the concept best. You should probably
study it's operation in a simple test program first.
The concept, IIRC, is as follows: One (or more) threads needs to wait
for a condition. I think in our case this is the main command thread,
and it needs to wait till worker thread has completed and has pushed the
IOHandler. Instead of having the main thread sleeping a fixed interval,
you let it sleep (possibly for a infinite timeout) and wake when a
condition occurs (e.g. a flag is set, or variable exceeds a particular
value). The other thread (e.g. the one pushing the IOHandler) then sets
the signal once the condition has arisen. The point is, we wait for a
signal, *not* a timeout - the timeout is just (sometimes) there for if
things go wrong, e.g. the condition is never met - and in that case this
timeout/error condition would need to be dealt with (by the waiting
thread) correctly (e.g. an error message preceding the printing of the
prompt).
Greg Clayton's reply to you suggests that you use
"lldb/Host/Predicate.h". I've not used this myself, but I took a quick
look and it looks like it wraps up the above idea. I *think* that using
this, the main thread (the one waiting) will call one of the Predicate
class's Waitxxx functions, and the other thread will call on of the
Setxxx when it's ok for the waiter to wake.
I'd suggest you get your head round conditional variables first, then
check out using "lldb/Host/Predicate.h" in the lldb code.
Hope this helps! Keep us posted.
Matt
Shawn Best 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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