diff --git a/pageserver/src/tenant/timeline.rs b/pageserver/src/tenant/timeline.rs
index 0586ec38c8..f09617849c 100644
--- a/pageserver/src/tenant/timeline.rs
+++ b/pageserver/src/tenant/timeline.rs
@@ -292,7 +292,7 @@ pub struct Timeline {
     pub initdb_lsn: Lsn,
 
     /// When did we last calculate the partitioning?
-    partitioning: Mutex<(KeyPartitioning, Lsn)>,
+    partitioning: tokio::sync::Mutex<(KeyPartitioning, Lsn)>,
 
     /// Configuration: how often should the partitioning be recalculated.
     repartition_threshold: u64,
@@ -1640,7 +1640,7 @@ impl Timeline {
                     // initial logical size is 0.
                     LogicalSize::empty_initial()
                 },
-                partitioning: Mutex::new((KeyPartitioning::new(), Lsn(0))),
+                partitioning: tokio::sync::Mutex::new((KeyPartitioning::new(), Lsn(0))),
                 repartition_threshold: 0,
 
                 last_received_wal: Mutex::new(None),
@@ -3354,30 +3354,34 @@ impl Timeline {
         flags: EnumSet<CompactFlags>,
         ctx: &RequestContext,
     ) -> anyhow::Result<(KeyPartitioning, Lsn)> {
-        {
-            let partitioning_guard = self.partitioning.lock().unwrap();
-            let distance = lsn.0 - partitioning_guard.1 .0;
-            if partitioning_guard.1 != Lsn(0)
-                && distance <= self.repartition_threshold
-                && !flags.contains(CompactFlags::ForceRepartition)
-            {
-                debug!(
-                    distance,
-                    threshold = self.repartition_threshold,
-                    "no repartitioning needed"
-                );
-                return Ok((partitioning_guard.0.clone(), partitioning_guard.1));
-            }
+        let Ok(mut partitioning_guard) = self.partitioning.try_lock() else {
+            // NB: there are two callers, one is the compaction task, of which there is only one per struct Tenant and hence Timeline.
+            // The other is the initdb optimization in flush_frozen_layer, used by `boostrap_timeline`, which runs before `.activate()`
+            // and hence before the compaction task starts.
+            anyhow::bail!("repartition() called concurrently, this should not happen");
+        };
+        if lsn < partitioning_guard.1 {
+            anyhow::bail!("repartition() called with LSN going backwards, this should not happen");
         }
+
+        let distance = lsn.0 - partitioning_guard.1 .0;
+        if partitioning_guard.1 != Lsn(0)
+            && distance <= self.repartition_threshold
+            && !flags.contains(CompactFlags::ForceRepartition)
+        {
+            debug!(
+                distance,
+                threshold = self.repartition_threshold,
+                "no repartitioning needed"
+            );
+            return Ok((partitioning_guard.0.clone(), partitioning_guard.1));
+        }
+
         let keyspace = self.collect_keyspace(lsn, ctx).await?;
         let partitioning = keyspace.partition(partition_size);
 
-        let mut partitioning_guard = self.partitioning.lock().unwrap();
-        if lsn > partitioning_guard.1 {
-            *partitioning_guard = (partitioning, lsn);
-        } else {
-            warn!("Concurrent repartitioning of keyspace. This unexpected, but probably harmless");
-        }
+        *partitioning_guard = (partitioning, lsn);
+
         Ok((partitioning_guard.0.clone(), partitioning_guard.1))
     }