Re: [PATCH v2] Make xmalloc and xrealloc thread-safe

> Nicolas Pitre <nico@fluxnic.net> wrote:
> > To avoid a deadlock if try_to_free_from_threads() is called while
> > read_lock is already locked within the same thread (may happen through
> > the read_sha1_file() path), a simple mutex ownership is added. This 
> > could have been handled automatically with the PTHREAD_MUTEX_RECURSIVE 
> > type but the Windows pthread emulation would get much more complex.
> ...
> > +static void try_to_free_from_threads(size_t size)
> > +{
> > +	int self = pthread_equal(read_mutex_owner, pthread_self());
> > +	if (!self)
> > +		read_lock();
> > +	release_pack_memory(size, -1);
> > +	if (!self)
> > +		read_unlock();
> > +}
> 
> Is there any concern that a partially unset read_mutex_owner might
> look like the current thread's identity?
> 
> That is, memset() can be setting the bytes one by one.  If the lock
> is being released we might observe the current owner as ourselves
> if we see only part of that release, and our identity is the same
> as another thread, only with the lower-address bytes unset.