Re: [Qemu-devel] [PATCH for-2.11] rcu: init globals only once

[Peter Xu <peterx@redhat.com>]

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