Re: [Qemu-devel] [PATCH] qemu_mutex_iothread_locked not correctly synchronized

[Paolo Bonzini]

On 24/11/2015 16:43, David Engraf wrote:
> Commit afbe70535ff1a8a7a32910cc15ebecc0ba92e7da introduced the function
> qemu_mutex_iothread_locked to avoid recursive locking of the iothread
> lock. The iothread_locked variable uses the __thread attribute which
> results in a per thread storage location whereas the qemu_global_mutex
> is not thread specific. This leads to race conditions because the
> variable is not in sync between threads.

Frankly, this makes no sense.  You're modifying the
qemu_mutex_iothread_locked function to return whether _some_ thread has
taken the mutex, but the function tells you whether _the calling_ thread
has taken the mutex (that's the point about recursive locking).  This
breaks the callers of qemu_mutex_iothread_locked completely.

The variable need not be in sync between the different threads; each
thread only needs to know about itself.  The current code works because:

1) the iothread mutex is not locked before qemu_mutex_lock_iothread

2) the iothread mutex is not locked after qemu_mutex_lock_iothread

3) qemu_cond_wait doesn't matter because the thread does not run during
a qemu_cond_wait.

> I triggered this problem reproducible on a Windows machine whereas Linux 
> works fine. 

What are the symptoms, and what is your Windows compiler?

Paolo