vips_foreign_load_generate( VipsRegion *or, 
	void *seq, void *a, void *b, gboolean *stop )
{
	VipsRegion *ir = (VipsRegion *) seq;

        VipsRect *r = &or->valid;

        /* Ask for input we need.
         */
        if( vips_region_prepare( ir, r ) )
                return( -1 );

        /* Attach output region to that.
         */
        if( vips_region_region( or, ir, r, r->left, r->top ) )
                return( -1 );

        return( 0 );
}

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.