diff --git a/src/backend/storage/ipc/ipc.c b/src/backend/storage/ipc/ipc.c
index 6df7413fe9b..cb944edd8df 100644
--- a/src/backend/storage/ipc/ipc.c
+++ b/src/backend/storage/ipc/ipc.c
@@ -29,6 +29,7 @@
 #endif
 #include "storage/dsm.h"
 #include "storage/ipc.h"
+#include "storage/lwlock.h"
 #include "tcop/tcopprot.h"
 
 
@@ -229,13 +230,19 @@ shmem_exit(int code)
 {
 	shmem_exit_inprogress = true;
 
+	/*
+	 * Release any LWLocks we might be holding before callbacks run. This
+	 * prevents accessing locks in detached DSM segments and allows callbacks
+	 * to acquire new locks.
+	 */
+	LWLockReleaseAll();
+
 	/*
 	 * Call before_shmem_exit callbacks.
 	 *
 	 * These should be things that need most of the system to still be up and
 	 * working, such as cleanup of temp relations, which requires catalog
-	 * access; or things that need to be completed because later cleanup steps
-	 * depend on them, such as releasing lwlocks.
+	 * access.
 	 */
 	elog(DEBUG3, "shmem_exit(%d): %d before_shmem_exit callbacks to make",
 		 code, before_shmem_exit_index);
diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c
index a133c97b992..517c55375b4 100644
--- a/src/backend/storage/lmgr/lwlock.c
+++ b/src/backend/storage/lmgr/lwlock.c
@@ -1883,6 +1883,10 @@ LWLockReleaseClearVar(LWLock *lock, pg_atomic_uint64 *valptr, uint64 val)
  * unchanged by this operation.  This is necessary since InterruptHoldoffCount
  * has been set to an appropriate level earlier in error recovery. We could
  * decrement it below zero if we allow it to drop for each released lock!
+ *
+ * Note that this function must be safe to call even before the LWLock
+ * subsystem has been initialized (e.g., during early startup failures).
+ * In that case, num_held_lwlocks will be 0 and we do nothing.
  */
 void
 LWLockReleaseAll(void)
@@ -1893,6 +1897,8 @@ LWLockReleaseAll(void)
 
 		LWLockRelease(held_lwlocks[num_held_lwlocks - 1].lock);
 	}
+
+	Assert(num_held_lwlocks == 0);
 }