That was the gap. A single misplaced memory barrier. A missing wmb() (write memory barrier) between writing the "I'm ready" flag and re-enabling interrupts.
The link came up. The drone's test pattern started: 10,000 packets per second, each with a sequence number. driver for pci device
But her eye caught a tiny race. An if statement that checked a flag before re-enabling interrupts from the chip. If that flag was set late—by even a microsecond—the chip would think the driver was still busy. It would stop raising interrupts. The receive ring would fill. Packets would sit in the chip's FIFO, getting old, then get dropped. That was the gap
But her network was dropping packets like a clumsy waiter. Every few thousand frames, a silent gap. A micro-death. And in her line of work—real-time sensor fusion for an autonomous vineyard drone—a dropped packet meant a crashed drone meant a ruined Pinot Noir harvest. The link came up
The file unfolded. Forty-thousand lines of C, a cathedral of bit twiddling and DMA descriptors. Elara began her liturgy.
Then she found it. The interrupt handler: rtl8169_interrupt . On a packet receive, the chip raises a line on the PCI bus. The kernel calls this routine. The routine reads a status register via MMIO. Then it does the dance: acknowledge the interrupt, schedule an NAPI poll, clear the bit.
"Of course," she whispered.