On 12/31/2014 07:19 PM, Rich Felker wrote: > On Tue, Dec 30, 2014 at 11:12:40PM -0500, Carlos O'Donell wrote: >> That is not the definition of reentrancy that I had in mind. > > Indeed. To me, "reentrancy" means the ability to enter the code again > while one or more instances of the same function are already running > in the same thread. In my view this doesn't necessarily include > asynchronous reentrancy unless explicitly stated, and it doesn't have > anything to do with the concept of reentrancy in the obsolete POSIX > 2001 that was removed and replaced with "thread safety" in later > editions of POSIX. > >> [snip] >> >>> Or are these questions meaningless because this definition is not the >>> one we want to use for Reentrancy? >> >> We already use AS-safe to indicate that a function is safe to reenter >> from a signal handler. > > AS-safety means more than that. It means that it's safe to use the > function from a signal handler no matter what function the signal > handler interrupted, and that it's safe to call any standard function > from a signal handler that interrupted the AS-safe function. Note that > the latter part can be vacuously true if AS-safety is achieved by > blocking signals for the duration of the function. > >> We already use MT-safe to indicate that a function is safe to reenter >> from another thread. >> >> The only definition of reentrancy I had in mind is that which is derived >> from the original definition of single-threaded program reetrancy, with >> the exclusion of signals. > > This agrees with my definition. > >> The reason I want to use this definition is to more formally describe >> those functions which are safe to call from a user provided malloc. >> A user provided malloc can be called from almost anywhere in glibc, it >> interrupts core glibc code, it only synchronously interrupts core >> glibc code (when malloc is called), and limiting a user provided malloc >> to AS-safe functions would be punative (though that is what we'll be >> doing in the initial documentation pass). >> >> Definition of reentrancy: >> ========================= >> ~~~~ >> A function is reentrant if the one thread may safely call >> the function before a previous call to the same function >> by the same thread completes, but need not be safe if the >> second or subsequent calls are made while handling a signal. >> ~~~~ >> Safely implies no data loss. >> >> By definition all AS-safe functions are reetrant. >> >> A reetrant function may be AS-unsafe and MT-unsafe. >> >> A reetrant function may be AS-unsafe and MT-safe. >> >> A reetrant function may be AS-safe (makes it reetrant by definition) and MT-unsafe. > > No, AS-safe does not imply reentrant. Consider a function that invokes > a callback but blocks signals during its execution. It's vacuously > AS-safe but instead of via a signal handler it can be re-entered via > the callback, and whether such reentrancy is safe is a non-trivial > question. You are correct. Thank you for the contradicting example. The correction should read: "A reentrant function may be AS-safe and MT-unsafe." Thus the safety note is entirely orthogonal to AS and MT safety. Cheers, Carlos. -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
