how to flush write to raspberry pi gpio pins - raspberry-pi3

Question

I have a program written in C++ on a Raspberry Pi 3. I mmap /dev/gpiomem to access the GPIO registers directly in user mode. Here is my function to write to output pins:
static uint32_t volatile *gpiopage; // initialized by mmap() of /dev/gpiomem
static uint32_t lastretries = 0;
void PhysLib::writegpio (uint32_t value)
{
uint32_t mask = 0xFFF000; // [11:00] input pins
// [23:12] output pins
// all the pins go through an inverter converting the 3.3V to 5V
gpiopage[GPIO_CLR0] = value;
gpiopage[GPIO_SET0] = ~ value;
// sometimes takes 1 or 2 retries to make sure signal gets out
uint32_t retries = 0;
while (true) {
uint32_t readback = ~ gpiopage[GPIO_LEV0];
uint32_t diff = (readback ^ value) & mask;
if (diff == 0) break;
if (++ retries > 1000) {
fprintf (stderr, "PhysLib::writegpio: wrote %08X mask %08X, readback %08X diff %08X\n",
value, mask, readback, diff);
abort ();
}
}
if (lastretries < retries) {
lastretries = retries;
printf ("PhysLib::writegpio: retries %u\n", retries);
}
}
Apparently there is some internal cache or something delaying the actual updating of the pins. So I'm wondering if there is some magic MRC or MCR or whatever that I can put so I don't have to read the pins to wait for the update to actually occur.
I'm quite sure this is happening because this code is part of a loop:
while (true) {
writegpio (0x800000); // set gpio pin 23
software timing loop for 1uS
writegpio (0); // clear gpio pin 23
software timing loop for 1uS
}
Sometimes Linux timeslices during the software timing loops on me and I get a delay longer than 1uS, which is ok for this project. Before I put in the code that loops until it reads the updated bit, sometimes the voltage on the pin stays high for longer than 1uS and is then low for correspondingly less than 1uS, or vice versa, implying that there is a total of 2uS delay for the two timing loops but the update of the actual pin is being delayed by the hardware. After inserting the corrective code, I always get at least 1uS of high voltage and 1uS of low voltage each time through the loop.