On 2019/10/17 上午7:33, Will Deacon wrote:
Hi all,
In an attempt to remove the remaining traces of [smp_]read_barrier_depends()
following my previous patches to strengthen READ_ONCE() for Alpha [1], I
ended up trying to decipher the read_barrier_depends() usage in the vhost
driver:
--->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...
Thanks,
I guess it was because we will read from the address stored in the iov like:
1) trasnlate_desc() that stores the userspace buffer pointer in the iov
2) copy_from_iter() that reads from those pointers
So we need a data dependency barrier in the middle as explained in the
memory-barriers.txt? (since READ_ONCE is not used in iov iterator).
Thanks
Will
[1] c2bc66082e10 ("locking/barriers: Add implicit smp_read_barrier_depends() to READ_ONCE()")
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