IB/core: Fix zero dmac race in neighbor resolution dst_fetch_ha() checks nud_state without holding the neighbor lock, then copies ha under the seqlock. A race in __neigh_update() where nud_state is set to NUD_REACHABLE before ha is written allows dst_fetch_ha() to read a zero MAC address while the seqlock reports no concurrent writer. netevent_callback amplifies this by waking ALL pending addr_req workers when ANY neighbor becomes NUD_VALID. At scale (N peers resolving ARP concurrently), the hit probability scales as N^2, making it near-certain for large RDMA workloads. N(A): neigh_update(A) W(A): addr_resolve(A) | [sleep] | write_lock_bh(&A->lock) | | A->nud_state = NUD_REACHABLE | | // A->ha is still 0 | | [woken by netevent_cb() of | another neighbour] | | dst_fetch_ha(A) | | A->nud_state & NUD_VALID | | read_seqbegin(&A->ha_lock) | | snapshot = A->ha /* 0 */ | | read_seqretry(&A->ha_lock) | | return snapshot | seqlock(&A->ha_lock) | A->ha = mac_A /* too late */ | sequnlock(&A->ha_lock) | write_unlock_bh(&A->lock) The incorrect/zero mac is read and programmed in the device QP while it was not yet updated. This causes silent packet loss and eventual RETRY_EXC_ERR. Fix by holding the neighbor read lock across the nud_state check and ha copy in dst_fetch_ha(), ensuring it synchronizes with __neigh_update() which is updating while holding the write lock. Signed-off-by: Chen Zhao <chezhao@nvidia.com> Reviewed-by: Parav Pandit <parav@nvidia.com> Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
diff --git a/drivers/infiniband/core/addr.c b/drivers/infiniband/core/addr.c index 8667466..6526fda 100644 --- a/drivers/infiniband/core/addr.c +++ b/drivers/infiniband/core/addr.c
@@ -321,11 +321,14 @@ static int dst_fetch_ha(const struct dst_entry *dst, if (!n) return -ENODATA; + read_lock_bh(&n->lock); if (!(n->nud_state & NUD_VALID)) { + read_unlock_bh(&n->lock); neigh_event_send(n, NULL); ret = -ENODATA; } else { neigh_ha_snapshot(dev_addr->dst_dev_addr, n, dst->dev); + read_unlock_bh(&n->lock); } neigh_release(n);