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

[Eric Blake]

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.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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