Petr Tesarik

The issue is related to a functional regression in the Linux kernel's swiotlb component, which causes double-allocation of slots due to broken alignment handling. This can lead to buffer corruption and/or a hang when using vsock in a virtual machine with a restricted DMA SWIOTLB pool. The problem occurs because swiotlb alloc() expects a page-aligned allocation but returns a pointer to the 'struct page' corresponding to the allocation, resulting in double-allocation of the first half of the 4KiB page. The issue is resolved by treating the allocation alignment separately from any additional alignment requirements from the device.

The issue is related to the protection of updates of 64-bit statistics counters in the Linux kernel. A comment in <linux/u64 stats sync.h> explains that the write side of struct u64 stats sync must ensure mutual exclusion, or one seqcount update could be lost on 32-bit platforms, thus blocking readers forever. Such lockups have been observed in real-world scenarios after stmmac xmit() on one CPU raced with stmmac napi poll tx() on another CPU. The fix involves splitting the statics into three parts: fields updated only under the tx queue lock, fields updated only during NAPI poll, and fields updated only from interrupt context.