diff --git a/pageserver/src/tenant/mgr.rs b/pageserver/src/tenant/mgr.rs
index 445676c4ae..09cedf9523 100644
--- a/pageserver/src/tenant/mgr.rs
+++ b/pageserver/src/tenant/mgr.rs
@@ -2063,6 +2063,7 @@ impl TenantManager {
 
         slot_guard.upsert(TenantSlot::Attached(tenant.clone()))?;
 
+        // finally ask the restarted tenant to complete the detach
         tenant
             .ongoing_timeline_detach
             .complete(attempt, &tenant)
diff --git a/pageserver/src/tenant/timeline/detach_ancestor.rs b/pageserver/src/tenant/timeline/detach_ancestor.rs
index fff33eee8b..f141c8510b 100644
--- a/pageserver/src/tenant/timeline/detach_ancestor.rs
+++ b/pageserver/src/tenant/timeline/detach_ancestor.rs
@@ -131,13 +131,20 @@ impl Default for Options {
 /// of each timeline tree.
 #[derive(Default)]
 pub(crate) struct SharedState {
-    inner: std::sync::Mutex<Option<(TimelineId, completion::Barrier)>>,
+    inner: std::sync::Mutex<SharedStateInner>,
+    gc_waiting: tokio::sync::Notify,
+    attempt_waiting: tokio::sync::Notify,
 }
 
 impl SharedState {
     /// Notify an uninitialized shared state that an attempt to detach timeline ancestor continues
     /// from previous instance.
-    pub(crate) fn continue_existing_attempt(&self, _attempt: &Attempt) {}
+    pub(crate) fn continue_existing_attempt(&self, attempt: &Attempt) {
+        self.inner
+            .lock()
+            .unwrap()
+            .continue_existing_attempt(attempt);
+    }
 
     /// Only GC must be paused while a detach ancestor is ongoing. Compaction can happen, to aid
     /// with any ongoing ingestion. Compaction even after restart is ok because layers will not be
@@ -145,23 +152,40 @@ impl SharedState {
     pub(crate) fn attempt_blocks_gc(&self) -> bool {
         // if we have any started and not finished ancestor detaches, we must remain paused
         // and also let any trying to start operation know that we've paused.
-
-        // Two cases:
-        // - there is an actual attempt started
-        // - we have learned from indexparts that an attempt will be retried in near future
-        self.inner.lock().unwrap().is_some()
+        self.mark_witnessed_and_notify()
     }
 
     /// Sleep for the duration, while letting any ongoing ancestor_detach attempt know that gc has
     /// been paused.
-    ///
-    /// Cancellation safe.
     pub(crate) async fn gc_sleeping_while<T, F: std::future::Future<Output = T>>(
         &self,
         fut: F,
     ) -> T {
         // this needs to wrap the sleeping so that we can quickly let ancestor_detach continue
-        fut.await
+        let mut fut = std::pin::pin!(fut);
+
+        loop {
+            tokio::select! {
+                x = &mut fut => { return x; },
+                _ = self.gc_waiting.notified() => {
+                    self.mark_witnessed_and_notify();
+                }
+            }
+        }
+    }
+
+    fn mark_witnessed_and_notify(&self) -> bool {
+        let mut g = self.inner.lock().unwrap();
+        if let Some((_, witnessed)) = g.latest.as_mut() {
+            if !*witnessed {
+                *witnessed = true;
+                self.attempt_waiting.notify_one();
+            }
+
+            true
+        } else {
+            false
+        }
     }
 
     /// Acquire the exclusive lock for a new detach ancestor attempt and ensure that GC task has
@@ -173,26 +197,60 @@ impl SharedState {
             return Err(Error::ShuttingDown);
         }
 
-        let (guard, barrier) = completion::channel();
-
-        {
+        let completion = {
             let mut guard = self.inner.lock().unwrap();
-            if let Some((tl, other)) = guard.as_ref() {
-                if !other.is_ready() {
-                    return Err(Error::OtherTimelineDetachOngoing(*tl));
-                }
-            }
-            *guard = Some((detached.timeline_id, barrier));
-        }
 
-        // FIXME: modify the index part to have a "detach-ancestor: inprogress { started_at }"
-        // unsure if it should be awaited to upload yet...
+            let completion = guard.start_new(&detached.timeline_id)?;
+
+            // now that we changed the contents, notify any long-sleeping gc
+            self.gc_waiting.notify_one();
+
+            completion
+        };
+
+        let started_at = std::time::Instant::now();
+
+        let mut cancelled = std::pin::pin!(detached.cancel.cancelled());
+
+        loop {
+            tokio::select! {
+                _ = &mut cancelled => { return Err(Error::ShuttingDown); },
+                _ = self.attempt_waiting.notified() => {
+                    // reading a notification which was not intended for us is not a problem,
+                    // because we check if *our* progress has been witnessed by gc.
+                },
+            };
+
+            let g = self.inner.lock().unwrap();
+            if g.is_gc_paused(&detached.timeline_id) {
+                break;
+            }
+        }
 
         // finally
         let gate_entered = detached.gate.enter().map_err(|_| Error::ShuttingDown)?;
 
+        let synced_in = started_at.elapsed();
+
+        detached
+            .remote_client
+            .schedule_started_detach_ancestor_mark_and_wait()
+            .await
+            // FIXME: aaaargh
+            .map_err(|_| Error::ShuttingDown)?;
+
+        let uploaded_in = started_at.elapsed() - synced_in;
+
+        // FIXME: get rid of this logging or make it a metric or two
+        tracing::info!(
+            sync_ms = synced_in.as_millis(),
+            upload_ms = uploaded_in.as_millis(),
+            "gc paused, gate entered, and uploaded"
+        );
+
         Ok(Attempt {
-            _guard: guard,
+            timeline_id: detached.timeline_id,
+            _guard: completion,
             gate_entered: Some(gate_entered),
         })
     }
@@ -204,22 +262,198 @@ impl SharedState {
     /// Cancellation safe.
     pub(crate) async fn complete(
         &self,
-        _attempt: Attempt,
-        _tenant: &Arc<Tenant>,
+        attempt: Attempt,
+        tenant: &Arc<Tenant>,
     ) -> Result<(), Error> {
         // do we need the tenant to actually activate...? yes we do, but that can happen via
         // background task starting, because we will similarly want to confirm that the gc has
         // paused, before we unpause it?
-        //
+
         // assert that such and such state has been collected
         // find the timeline the attempt represents
         // using the timelines remote client, upload an index part with completion information
+
+        {
+            let g = self.inner.lock().unwrap();
+
+            // TODO: cover the case where retry completes?
+            g.validate(&attempt);
+        }
+
+        let attempt = scopeguard::guard(attempt, |attempt| {
+            // our attempt will no longer be valid, so release it
+            self.inner.lock().unwrap().cancel(attempt);
+        });
+
+        // no failpoint needed here, because the next one is the first mutating
+
+        tenant
+            .wait_to_become_active(std::time::Duration::from_secs(9999))
+            .await
+            .map_err(Error::WaitToActivate)?;
+
+        let Some(timeline) = tenant
+            .timelines
+            .lock()
+            .unwrap()
+            .get(&attempt.timeline_id)
+            .cloned()
+        else {
+            unreachable!("unsure if there is an ordering, but perhaps this is possible?");
+        };
+
+        // this should be an 503 at least...?
+        fail::fail_point!(
+            "timeline-detach-ancestor::complete_before_uploading",
+            |_| Err(Error::Failpoint(
+                "timeline-detach-ancestor::complete_before_uploading"
+            ))
+        );
+
+        timeline
+            .remote_client
+            .schedule_completed_detach_ancestor_mark_and_wait()
+            .await
+            .map_err(|_| Error::ShuttingDown)?;
+
+        // now that the upload has gone through, we must remove this timeline from inprogress
+        let attempt = scopeguard::ScopeGuard::into_inner(attempt);
+
+        self.inner.lock().unwrap().complete(attempt);
+
         Ok(())
     }
 }
 
-/// Token which represents a persistent, exclusive, awaitable single attempt.
+#[derive(Default)]
+struct SharedStateInner {
+    known_ongoing: std::collections::HashSet<TimelineId>,
+    latest: Option<(ExistingAttempt, bool)>,
+}
+
+impl SharedStateInner {
+    fn continue_existing_attempt(&mut self, attempt: &Attempt) {
+        assert!(self.known_ongoing.is_empty());
+        assert!(self.latest.is_none());
+        self.known_ongoing.insert(attempt.timeline_id);
+        self.latest = Some((
+            ExistingAttempt::ContinuedOverRestart(attempt.timeline_id),
+            false,
+        ));
+    }
+
+    fn start_new(&mut self, detached: &TimelineId) -> Result<completion::Completion, Error> {
+        let completion = if let Some((existing, witnessed)) = self.latest.as_mut() {
+            let completion = existing.start_new(detached)?;
+            *witnessed = false;
+            completion
+        } else {
+            let (completion, attempt) = ExistingAttempt::new(detached);
+            self.latest = Some((attempt, false));
+            completion
+        };
+
+        self.known_ongoing.insert(*detached);
+
+        Ok(completion)
+    }
+
+    fn is_gc_paused(&self, timeline_id: &TimelineId) -> bool {
+        match &self.latest {
+            Some((ExistingAttempt::Actual(x, _), paused)) => {
+                assert_eq!(x, timeline_id);
+                *paused
+            }
+            other => {
+                unreachable!("unexpected state {other:?}")
+            }
+        }
+    }
+
+    fn validate(&self, attempt: &Attempt) {
+        match self.latest.as_ref() {
+            Some((ExistingAttempt::ContinuedOverRestart(x), _)) if x == &attempt.timeline_id => {
+                assert!(self.known_ongoing.contains(&attempt.timeline_id));
+            }
+            other => unreachable!("unexpected: {other:?}"),
+        }
+    }
+
+    fn complete(&mut self, attempt: Attempt) {
+        match self.latest.as_ref() {
+            Some((ExistingAttempt::ContinuedOverRestart(x), witnessed))
+                if x == &attempt.timeline_id =>
+            {
+                let witnessed = *witnessed;
+                assert!(self.known_ongoing.remove(&attempt.timeline_id));
+
+                if self.known_ongoing.is_empty() {
+                    self.latest = None;
+                    tracing::info!("gc is now unblocked");
+                } else {
+                    self.latest = Some((ExistingAttempt::ReadFromIndexPart, witnessed));
+                }
+            }
+            other => unreachable!("unexpected: {other:?}"),
+        }
+    }
+
+    fn cancel(&mut self, attempt: Attempt) {
+        match self.latest.as_ref() {
+            Some((ExistingAttempt::ContinuedOverRestart(x), witnessed))
+                if x == &attempt.timeline_id =>
+            {
+                let witnessed = *witnessed;
+                assert!(!self.known_ongoing.is_empty());
+                self.latest = Some((ExistingAttempt::ReadFromIndexPart, witnessed));
+            }
+            other => unreachable!("unexpected: {other:?}"),
+        }
+    }
+}
+
+#[derive(Debug)]
+enum ExistingAttempt {
+    /// Informative; there are still non-zero known ongoing timeline ancestor detaches
+    ReadFromIndexPart,
+
+    /// Exclusive lock carried over tenant reset
+    ContinuedOverRestart(TimelineId),
+
+    /// Exclusive while barrier shows the task running
+    Actual(TimelineId, completion::Barrier),
+}
+
+impl ExistingAttempt {
+    fn start_new(&mut self, detached: &TimelineId) -> Result<completion::Completion, Error> {
+        use ExistingAttempt::*;
+        match self {
+            ReadFromIndexPart => {}
+            Actual(other, barrier) if barrier.is_ready() => {
+                if other != detached {
+                    tracing::warn!(prev=%other, next=%detached, "switching ongoing detach; this is not expected to happen normally, but doesn't necessarily mean anything catastrophic");
+                }
+            }
+            Actual(other, _) | ContinuedOverRestart(other) => {
+                return Err(Error::OtherTimelineDetachOngoing(*other))
+            }
+        }
+
+        let (guard, attempt) = Self::new(detached);
+        *self = attempt;
+        Ok(guard)
+    }
+
+    fn new(detached: &TimelineId) -> (completion::Completion, Self) {
+        let (guard, barrier) = completion::channel();
+        (guard, ExistingAttempt::Actual(*detached, barrier))
+    }
+}
+
+/// Represents an across tenant reset exclusive single attempt to detach ancestor.
 pub(crate) struct Attempt {
+    timeline_id: TimelineId,
+
     _guard: completion::Completion,
     gate_entered: Option<utils::sync::gate::GateGuard>,
 }
@@ -232,20 +466,36 @@ impl Attempt {
 }
 
 #[derive(Default)]
-pub(crate) struct SharedStateBuilder {}
+pub(crate) struct SharedStateBuilder {
+    inprogress: std::collections::HashSet<TimelineId>,
+}
 
 impl SharedStateBuilder {
     /// While loading, visit a timelines persistent [`crate::tenant::IndexPart`] and record if it is being
     /// detached.
     pub(crate) fn record_loading_timeline(
         &mut self,
-        _timeline_id: &TimelineId,
-        _index_part: &crate::tenant::IndexPart,
+        timeline_id: &TimelineId,
+        index_part: &crate::tenant::IndexPart,
     ) {
+        if index_part.ongoing_detach_ancestor.is_some() {
+            // if the loading a timeline fails, tenant loading must fail as it does right now, or
+            // something more elaborate needs to be done with this tracking
+            self.inprogress.insert(*timeline_id);
+        }
     }
 
     /// Merge the loaded not yet deleting in-progress to the existing datastructure.
-    pub(crate) fn build(self, _target: &SharedState) {}
+    pub(crate) fn build(self, target: &SharedState) {
+        let mut g = target.inner.lock().unwrap();
+
+        assert_eq!(g.latest.is_none(), g.known_ongoing.is_empty());
+
+        g.known_ongoing.extend(self.inprogress.into_iter());
+        if g.latest.is_none() && !g.known_ongoing.is_empty() {
+            g.latest = Some((ExistingAttempt::ReadFromIndexPart, false));
+        }
+    }
 }
 
 /// See [`Timeline::prepare_to_detach_from_ancestor`]