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,
Will
[1] c2bc66082e10 ("locking/barriers: Add implicit
smp_read_barrier_depends() to READ_ONCE()")
[Bah: I typoed the LKML address, so I've fixed it for this one] On Thu, Oct 17, 2019 at 12:33:40AM +0100, 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, > > Will > > [1] c2bc66082e10 ("locking/barriers: Add implicit smp_read_barrier_depends() to READ_ONCE()")
On 2019/10/17 ??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()")
On Thu, Oct 17, 2019 at 10:17:18AM +0800, Jason Wang wrote:> On 2019/10/17 ??7:33, Will Deacon wrote: > > 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... > > > > 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 pointersIsn't that exactly the same flow as vhost_copy_from_user(), which doesn't have the barrier? Staring at the code some more, my best bet at the moment is that the load of 'indirect->addr' is probably the one to worry about, since it's part of the vring and can be updated concurrently.> 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).If the barrier is actually required, then there must be a concurrent access involved, in which case READ_ONCE should also be used. So I would propose something like the diff below, but I'd still be glad to hear whether I'm barking up the wrong tree. Will --->8 diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c index 36ca2cf419bf..2e370a229fea 100644 --- a/drivers/vhost/vhost.c +++ b/drivers/vhost/vhost.c @@ -2107,6 +2107,7 @@ static int get_indirect(struct vhost_virtqueue *vq, { struct vring_desc desc; unsigned int i = 0, count, found = 0; + __virtio64 addr = READ_ONCE(indirect->addr); u32 len = vhost32_to_cpu(vq, indirect->len); struct iov_iter from; int ret, access; @@ -2120,7 +2121,7 @@ static int get_indirect(struct vhost_virtqueue *vq, return -EINVAL; } - ret = translate_desc(vq, vhost64_to_cpu(vq, indirect->addr), len, vq->indirect, + ret = translate_desc(vq, vhost64_to_cpu(vq, addr), len, vq->indirect, UIO_MAXIOV, VHOST_ACCESS_RO); if (unlikely(ret < 0)) { if (ret != -EAGAIN) @@ -2129,10 +2130,6 @@ static int get_indirect(struct vhost_virtqueue *vq, } iov_iter_init(&from, READ, vq->indirect, ret, len); - /* We will use the result as an address to read from, so most - * architectures only need a compiler barrier here. */ - read_barrier_depends(); - count = len / sizeof desc; /* Buffers are chained via a 16 bit next field, so * we can have at most 2^16 of these. */ @@ -2152,12 +2149,12 @@ static int get_indirect(struct vhost_virtqueue *vq, } if (unlikely(!copy_from_iter_full(&desc, sizeof(desc), &from))) { vq_err(vq, "Failed indirect descriptor: idx %d, %zx\n", - i, (size_t)vhost64_to_cpu(vq, indirect->addr) + i * sizeof desc); + i, (size_t)vhost64_to_cpu(vq, addr) + i * sizeof desc); return -EINVAL; } if (unlikely(desc.flags & cpu_to_vhost16(vq, VRING_DESC_F_INDIRECT))) { vq_err(vq, "Nested indirect descriptor: idx %d, %zx\n", - i, (size_t)vhost64_to_cpu(vq, indirect->addr) + i * sizeof desc); + i, (size_t)vhost64_to_cpu(vq, addr) + i * sizeof desc); return -EINVAL; }
Will Deacon <will at kernel.org> 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, -- Email: Herbert Xu <herbert at gondor.apana.org.au> Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt