On Tue, Aug 08, 2017 at 09:26:43AM +0200, Paolo Bonzini wrote: > On 08/08/2017 09:00, Peter Xu wrote: > > We were calling rcu_init_complete() twice in the child processes when > > fork happened. However the pthread library does not really suggest to do > > it that way: > > > > http://pubs.opengroup.org/onlinepubs/7908799/xsh/pthread_mutex_init.html > > > > "Attempting to initialise an already initialised mutex results in > > undefined behaviour." > > > > Actually, IMHO we can do it in a more natural way: Firstly, we only init > > the RCU globals once in rcu_init(). Then, in rcu_init_child(), we unlock > > all the locks held in rcu_init_lock() just like what we do in the parent > > process, then do the rest of RCU re-init (e.g., create the RCU thread). > > 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). 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? > > Paolo -- Peter Xu
