On 2019/12/18 下午5:19, Herbert Xu wrote:
Will Deacon <will@xxxxxxxxxx> wrote:--->8 // drivers/vhost/vhost.c static int get_indirect(struct vhost_virtqueue *vq, struct iovec iov[], unsigned int iov_size, unsigned int *out_num, unsigned int *in_num, struct vhost_log *log, unsigned int *log_num, struct vring_desc *indirect) { [...] /* We will use the result as an address to read from, so most * architectures only need a compiler barrier here. */ read_barrier_depends(); --->8 Unfortunately, although the barrier is commented (hurrah!), it's not particularly enlightening about the accesses making up the dependency chain, and I don't understand the supposed need for a compiler barrier either (read_barrier_depends() doesn't generally provide this). Does anybody know which accesses are being ordered here? Usually you'd need a READ_ONCE()/rcu_dereference() beginning the chain, but I haven't managed to find one...I think what it's trying to separate is using indirect->addr as a base and then reading from that through copy_from_iter. Cheers,
The question is that there's a smp_rmb() before in vhost_get_vq_desc(), isn't it sufficient to do this?
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