On Tue, Aug 08, 2017 at 09:09:23AM -0500, Eric Blake wrote: > On 08/08/2017 02:49 AM, Peter Xu wrote: > >> This doesn't work for error-checking mutexes: rcu_init_child has a > >> different PID than the parent, so the mutexes aren't unlocked. It's > >> also true that right now we don't use error-checking mutexes (commit > >> 24fa90499f, "qemu-thread: do not use PTHREAD_MUTEX_ERRORCHECK", > >> 2015-03-10); however, that's also a bit sad. > >> > >> The reason for the undefined behavior is probably that some operating > >> systems allocate memory in pthread_mutex_init, and initializing twice > >> causes a memory leak. One such operating system is OpenBSD. :( > > > > Good to know. :) > > > > I thought pthread_atfork() was designed to solve such a locking > > problem (in child hanlder, we unlock all the held locks). > > What's also sad is that POSIX says that pthread_atfork() is rather > useless - there's no way it can be reliably used to do everything that > everyone wants (and I think this case of error-checking mutexes is just > ONE of those reasons). > > > If > > PTHREAD_MUTEX_ERRORCHECK cannot coop well with it, not sure whether > > that means we should just avoid using PTHREAD_MUTEX_ERRORCHECK in such > > a use case (but we should be able to use the error checks in other > > mutexes that do not need extra fork handling)? > > > > Another idea is: can we just destroy the mutex first then re-init it > > in subprocess? A quick glance in libpthread code shows that at least > > pthread_mutex_destroy() won't check PTHREAD_MUTEX_ERRORCHECK. > > > > Thanks, > > > >> > >> Eric, you chimed in on the patch that became commit 24fa90499f, what do > >> you suggest? > > If, after forking, you can successfully destroy the mutex to then > reinitialize it (even though you can't unlock it), then that sounds as > good as anything I can come up with. > > An alternative approach might be to add a new mutex that anyone obtains > just before forking; as long as you hold that mutex, you can then > release any other mutex, fork, and then reobtain in the parent - but it > still becomes tricky bookkeeping to know which locks need to be dropped > and reobtained, and I worry that gating fork performance with such a > heavy lock will have noticeable slowdowns.
It sounds like a MBQL (Much Bigger Qemu Lock :-). I am thinking whether we can simplify this. After all, we have existing assumptions: 1. for "-daemonize", we need the pthread_atfork() thing to make sure RCU works well even in the daemonized process 2. for all the rest of the fork cases, we assume that it will always be a quick exec() afterward, so need to maintain memory consistency Then, why not we just initialize RCU after os_daemonize()? IIUC, we can throw pthread_atfork() away then (considering that even it is not suggested to use "officially"). I guess it also depends on whether we'll need RCU before os_daemonize(). I assume not, but I may be wrong. Thanks, -- Peter Xu
