On 2011/02/28 05:26, Yuri wrote: > Forwarding to standards@ and davidxu@ per Garrett Cooper suggestion. > > Also I want to add that I came to this question while observing behavior > consistent with multiple wakeup on FreeBSD-8.1. The heavily > multi-threaded code that assumes that only one thread can be woken up by > one pthread_cond_signal call crashes, and the only reasonable > explanation so far is that more than one threads are actually being > woken up. > > Yuri > > > On 02/27/2011 12:54, Yuri wrote: >> On FreeBSD-8.1 this page says: >> The pthread_cond_signal() function unblocks one thread waiting for the >> condition variable cond. >> >> On Linux it says: >> The /pthread_cond_signal/() function shall unblock at least one of the >> threads that are blocked on the specified condition variable /cond/ >> (if any threads are blocked on /cond/). >> >> Also HP page >> (http://docs.hp.com/en/B2355-90130/pthread_cond_signal.3T.html) says: >> "If there are no threads blocked on /cond/, this function has no >> effect." And later it says: "It is possible that more than one thread >> can be unblocked due to a spurious wakeup." >> >> This is quite confusing: in case nobody is waiting does it block or >> not? In case other threads are waiting it's really "any arbitrary >> number of threads are woken up"? Or on FreeBSD it's strictly 1? >> Shouldn't this be defined in one and only way by POSIX and all >> POSIX-compliant systems should work exactly the same. >> >> I think man page should be expanded to give more comprehensive >> explanation. >> >> Yuri >> _______________________________________________ >> freebsd-hackers@freebsd.org mailing list >> http://lists.freebsd.org/mailman/listinfo/freebsd-hackers >> To unsubscribe, send any mail to >> "freebsd-hackers-unsubscr...@freebsd.org" >>
It is really not important if pthread_cond_signal wake up one thread or multiple threads in corner case, because POSIX said: --- When using condition variables there is always a Boolean predicate involving shared variables associated with each condition wait that is true if the thread should proceed. Spurious wakeups from the pthread_cond_timedwait() or pthread_cond_wait() functions may occur. Since the return from pthread_cond_timedwait() or pthread_cond_wait() does not imply anything about the value of this predicate, the predicate should be re-evaluated upon such return. --- I think in normal case, pthread_cond_signal will wake up one thread, but other events for example, UNIX signal and fork() may interrupt a thread sleeping in kernel, and cause pthread_cond_wait to return to userland, this is called spurious wakeup, and other events, I can not think of yet, but I believe they exist. Regards, David Xu _______________________________________________ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "freebsd-hackers-unsubscr...@freebsd.org"
