> On Sep 26, 2015, at 3:33 PM, Jerry Krinock <je...@ieee.org> wrote:
>
> In a OS X app, predating Grand Central Dispatch, in the main thread, I create
> and start a new thread
>
> NSThread* worker ;
> worker = [[NSThread alloc] initWithTarget:instance
> selector:@selector(beginWithInfo:)
> object:info] ;
> [worker setName:@“My Worker thread"] ;
> [worker start] ;
>
> usually this works OK, but sometimes, possibly when the above code executes a
> second or third time in rapid succession, [worker start] will block forever:
>
> Thread 1 Queue : com.apple.main-thread (serial)
> #0 0x00007fff93c50cd2 in semaphore_wait_trap ()
> #1 0x00000001007614b4 in _dispatch_semaphore_wait_slow ()
> #2 0x00007fff9134fae2 in -[NSThread start] ()
>
> Looking at the other threads, I see that “My Worker thread” (see Thread 455,
> below) has been created, and seems to be stuck while trying to *exit*, which
> I think is weird because if -[NSThread start] blocked until the new thread
> exitted, that would defeat the purpose of multithreading. Same thing if,
> say, My Worker thread executed some kind of “do this on the main thread” call.
>
> Should not -[NSThread start] always return before running any of my code in
> the new thread? So how could this happen? I’m running OS X 10.11.
The threads listed are all waiting for a spinlock used by the debugging tools.
(Specifically, it's the machinery that records stack traces of queue
operations.)
If you see this again, please capture a spindump and file a bug report.
After you file the bug report, you might be able to work around it like this:
defaults write com.apple.dt.Xcode DBGSkipRecordedFrames -bool YES
then quit and relaunch Xcode.
--
Greg Parker gpar...@apple.com Runtime Wrangler
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