89. Circular Buffer-I

Question.7

A UART ISR pushes received bytes, and the main loop pops them. Both access the same circular buffer:

// ISR (runs on UART interrupt)
void UART_ISR() {
   buffer_push(&rx_buf, UART_DR);
}

// Main loop
while (1) {
   if (!buffer_is_empty(&rx_buf)) {
       buffer_pop(&rx_buf, &byte);
       process(byte);
   }
}

What concurrency problem can occur?

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Nothing — the ISR and main loop access different indices (head vs tail)
The ISR can fire between the empty-check and the pop, corrupting count because both modify it without synchronization
The ISR overwrites main loop's local variables
The UART_DR read is invalid inside an ISR