Hi David, On Fri, Apr 20, 2012 at 4:44 PM, David Henningsson <david.henningsson at canonical.com> wrote: > While researching a bug I came across something that might be a bug in the > pa_once logic, but this stuff is tricky, so I might also be missing > something. > > Imagine this: > > ?* Thread 1 runs pa_once_begin, succeeds and starts running the payload (i e > the code that should only run once). > ?* Thread 2 starts running pa_once_begin, but only the first row. We're now > right *before* pa_atomic_inc(&control->ref) but *after* > pa_atomic_load(&control->done). > ?* Thread 1 finishes the payload, runs pa_once_done which sets control->done > and frees the mutex. > ?* Thread 2 continues, pa_once_begin succeeds and the payload is now run a > second time! > After reading your mail, I made some experiments by adding a usleep() call in Thread 1 between pa_atomic_load(&control->done) and pa_atomic_inc(&control->ref) and that failed once-test 100% of time. I reverted the usleep and made another experiment using 50000 iterations in once-test and it just failed. Good catch ! Fr?d?ric
