diff --git a/crates/api-core/src/handlers/machine_discovery.rs b/crates/api-core/src/handlers/machine_discovery.rs
index 04c86e3e7f..ae973b58cd 100644
--- a/crates/api-core/src/handlers/machine_discovery.rs
+++ b/crates/api-core/src/handlers/machine_discovery.rs
@@ -116,6 +116,12 @@ pub(crate) async fn discover_machine(
     };
 
     let mut txn = api.txn_begin().await?;
+    if hardware_info.is_dpu() {
+        // Discovery updates machine-interface rows before reconciliation. Take the segment locks
+        // first so the outer transaction preserves the allocator's advisory-lock-before-row-lock
+        // order instead of relying on reconciliation's inner savepoint to establish it later.
+        db::machine_interface::load_and_lock_all_admin_segments(&mut txn).await?;
+    }
     tracing::debug!(
         ?remote_ip,
         ?interface_id,
diff --git a/crates/api-core/src/handlers/managed_host.rs b/crates/api-core/src/handlers/managed_host.rs
index 9762f87deb..7bb03d7b87 100644
--- a/crates/api-core/src/handlers/managed_host.rs
+++ b/crates/api-core/src/handlers/managed_host.rs
@@ -283,6 +283,10 @@ async fn set_primary_interface_core(
     if current_primary_is_admin {
         db::machine_interface::reconcile_admin_addresses_for_host(&mut txn, &host_machine_id)
             .await?;
+    } else {
+        // The repair path has no current admin primary to reconcile, but it still must acquire
+        // segment locks before the primary-interface row updates below.
+        db::machine_interface::load_and_lock_all_admin_segments(&mut txn).await?;
     }
 
     // update the primary interface: clear the old primary (if any), then set the new.
diff --git a/crates/api-core/src/tests/set_primary_interface.rs b/crates/api-core/src/tests/set_primary_interface.rs
index 77c2a69c92..46bae4f71c 100644
--- a/crates/api-core/src/tests/set_primary_interface.rs
+++ b/crates/api-core/src/tests/set_primary_interface.rs
@@ -402,14 +402,64 @@ async fn test_set_primary_interface_repairs_dpu_host_with_no_admin_primary(
         .execute(&env.pool)
         .await?;
 
-    // Promoting the Admin interface must succeed (repair), not error after the BMC call.
-    env.api
-        .set_primary_interface(tonic::Request::new(forge::SetPrimaryInterfaceRequest {
+    // Hold the admin segment lock while the repair starts. The handler must wait for this lock
+    // before updating the interface row, or it can deadlock with DPU discovery's segment-first
+    // transaction.
+    let mut blocker_txn = db::Transaction::begin(&env.pool).await?;
+    let blocker_pid = sqlx::query_scalar::<_, i32>("SELECT pg_backend_pid()")
+        .fetch_one(blocker_txn.as_pgconn())
+        .await?;
+    db::machine_interface::load_and_lock_all_admin_segments(&mut blocker_txn).await?;
+
+    let api = env.api.clone();
+    let set_primary_task = tokio::spawn(async move {
+        api.set_primary_interface(tonic::Request::new(forge::SetPrimaryInterfaceRequest {
             host_machine_id: Some(host_id),
             interface_id: Some(promote_id),
             reboot: false,
         }))
-        .await?;
+        .await
+    });
+
+    let wait_result = tokio::time::timeout(std::time::Duration::from_secs(5), async {
+        loop {
+            let waiting: bool = sqlx::query_scalar(
+                "SELECT EXISTS (\
+                     SELECT 1 FROM pg_locks \
+                     WHERE locktype = 'advisory' AND NOT granted \
+                       AND $1 = ANY(pg_blocking_pids(pid))\
+                 )",
+            )
+            .bind(blocker_pid)
+            .fetch_one(&env.pool)
+            .await?;
+            if waiting {
+                return Ok::<(), sqlx::Error>(());
+            }
+            tokio::task::yield_now().await;
+        }
+    })
+    .await
+    .map_err(|_| {
+        std::io::Error::other("set-primary-interface did not wait for the admin segment lock")
+    });
+
+    let row_lock_result =
+        sqlx::query("SELECT id FROM machine_interfaces WHERE id = $1::uuid FOR UPDATE NOWAIT")
+            .bind(promote_id)
+            .fetch_one(&env.pool)
+            .await;
+
+    blocker_txn.commit().await?;
+    let set_primary_result = set_primary_task.await?;
+
+    wait_result??;
+    row_lock_result.map_err(|err| {
+        std::io::Error::other(format!(
+            "set-primary-interface locked the target row before the admin segment: {err}",
+        ))
+    })?;
+    set_primary_result?;
 
     // The promoted interface is now the only primary.
     let primaries_now: Vec<_> = {
diff --git a/crates/api-db/src/machine_interface.rs b/crates/api-db/src/machine_interface.rs
index 09e1ad1e53..e6d237704a 100644
--- a/crates/api-db/src/machine_interface.rs
+++ b/crates/api-db/src/machine_interface.rs
@@ -2627,7 +2627,7 @@ FOR UPDATE"#;
 /// This intentionally locks more broadly than the specific host touches so reconciliation follows
 /// the same high-level lock order as address allocation: segment advisory lock first, then
 /// machine interface/address row locks.
-async fn load_and_lock_all_admin_segments(
+pub async fn load_and_lock_all_admin_segments(
     txn: &mut Transaction<'_>,
 ) -> DatabaseResult<Vec<NetworkSegment>> {
     let mut segment_ids =