diff --git a/mmtk/Cargo.lock b/mmtk/Cargo.lock
index 4f8bd0d..fa4227b 100644
--- a/mmtk/Cargo.lock
+++ b/mmtk/Cargo.lock
@@ -93,9 +93,9 @@ checksum = "ace50bade8e6234aa140d9a2f552bbee1db4d353f69b8217bc503490fc1a9f26"
 
 [[package]]
 name = "bitflags"
-version = "2.6.0"
+version = "2.7.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b048fb63fd8b5923fc5aa7b340d8e156aec7ec02f0c78fa8a6ddc2613f6f71de"
+checksum = "1be3f42a67d6d345ecd59f675f3f012d6974981560836e938c22b424b85ce1be"
 
 [[package]]
 name = "built"
@@ -123,7 +123,7 @@ checksum = "3fa76293b4f7bb636ab88fd78228235b5248b4d05cc589aed610f954af5d7c7a"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.95",
+ "syn 2.0.96",
 ]
 
 [[package]]
@@ -219,7 +219,7 @@ checksum = "4e018fccbeeb50ff26562ece792ed06659b9c2dae79ece77c4456bb10d9bf79b"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.95",
+ "syn 2.0.96",
 ]
 
 [[package]]
@@ -230,7 +230,7 @@ checksum = "97369cbbc041bc366949bc74d34658d6cda5621039731c6310521892a3a20ae0"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.95",
+ "syn 2.0.96",
 ]
 
 [[package]]
@@ -262,7 +262,7 @@ checksum = "f282cfdfe92516eb26c2af8589c274c7c17681f5ecc03c18255fe741c6aa64eb"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.95",
+ "syn 2.0.96",
 ]
 
 [[package]]
@@ -449,7 +449,7 @@ checksum = "1ec89e9337638ecdc08744df490b221a7399bf8d164eb52a665454e60e075ad6"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.95",
+ "syn 2.0.96",
 ]
 
 [[package]]
@@ -584,7 +584,7 @@ dependencies = [
 [[package]]
 name = "mmtk"
 version = "0.30.0"
-source = "git+https://github.com/mmtk/mmtk-core.git?rev=ec745353a8de72b645613e0fef3ab7f5f1ad9bd1#ec745353a8de72b645613e0fef3ab7f5f1ad9bd1"
+source = "git+https://github.com/mmtk/mmtk-core.git?rev=68bf1b638263b250b12e55ef25bf8d09b01ca0b0#68bf1b638263b250b12e55ef25bf8d09b01ca0b0"
 dependencies = [
  "atomic",
  "atomic-traits",
@@ -620,12 +620,12 @@ dependencies = [
 [[package]]
 name = "mmtk-macros"
 version = "0.30.0"
-source = "git+https://github.com/mmtk/mmtk-core.git?rev=ec745353a8de72b645613e0fef3ab7f5f1ad9bd1#ec745353a8de72b645613e0fef3ab7f5f1ad9bd1"
+source = "git+https://github.com/mmtk/mmtk-core.git?rev=68bf1b638263b250b12e55ef25bf8d09b01ca0b0#68bf1b638263b250b12e55ef25bf8d09b01ca0b0"
 dependencies = [
  "proc-macro-error",
  "proc-macro2",
  "quote",
- "syn 2.0.95",
+ "syn 2.0.96",
 ]
 
 [[package]]
@@ -835,7 +835,7 @@ checksum = "5a9bf7cf98d04a2b28aead066b7496853d4779c9cc183c440dbac457641e19a0"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.95",
+ "syn 2.0.96",
 ]
 
 [[package]]
@@ -887,7 +887,7 @@ dependencies = [
  "proc-macro2",
  "quote",
  "rustversion",
- "syn 2.0.95",
+ "syn 2.0.96",
 ]
 
 [[package]]
@@ -902,9 +902,9 @@ dependencies = [
 
 [[package]]
 name = "syn"
-version = "2.0.95"
+version = "2.0.96"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "46f71c0377baf4ef1cc3e3402ded576dccc315800fbc62dfc7fe04b009773b4a"
+checksum = "d5d0adab1ae378d7f53bdebc67a39f1f151407ef230f0ce2883572f5d8985c80"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -919,7 +919,7 @@ checksum = "c8af7666ab7b6390ab78131fb5b0fce11d6b7a6951602017c35fa82800708971"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.95",
+ "syn 2.0.96",
 ]
 
 [[package]]
@@ -1149,7 +1149,7 @@ checksum = "2380878cad4ac9aac1e2435f3eb4020e8374b5f13c296cb75b4620ff8e229154"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.95",
+ "syn 2.0.96",
  "synstructure",
 ]
 
@@ -1170,7 +1170,7 @@ checksum = "595eed982f7d355beb85837f651fa22e90b3c044842dc7f2c2842c086f295808"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.95",
+ "syn 2.0.96",
  "synstructure",
 ]
 
@@ -1193,5 +1193,5 @@ checksum = "6eafa6dfb17584ea3e2bd6e76e0cc15ad7af12b09abdd1ca55961bed9b1063c6"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.95",
+ "syn 2.0.96",
 ]
diff --git a/mmtk/Cargo.toml b/mmtk/Cargo.toml
index dca7024..5d5b5e3 100644
--- a/mmtk/Cargo.toml
+++ b/mmtk/Cargo.toml
@@ -37,7 +37,7 @@ features = ["is_mmtk_object", "object_pinning", "sticky_immix_non_moving_nursery
 
 # Uncomment the following lines to use mmtk-core from the official repository.
 git = "https://github.com/mmtk/mmtk-core.git"
-rev = "ec745353a8de72b645613e0fef3ab7f5f1ad9bd1"
+rev = "68bf1b638263b250b12e55ef25bf8d09b01ca0b0"
 
 # Uncomment the following line to use mmtk-core from a local repository.
 #path = "../../mmtk-core"
diff --git a/mmtk/src/scanning.rs b/mmtk/src/scanning.rs
index 40d771f..c953e44 100644
--- a/mmtk/src/scanning.rs
+++ b/mmtk/src/scanning.rs
@@ -10,6 +10,11 @@ use mmtk::{Mutator, MutatorContext};
 pub struct VMScanning {}
 
 impl Scanning<Ruby> for VMScanning {
+    /// CRuby may do some non-thread-safe operations, such as cleaning up the call cache, while
+    /// scanning an object.  We force each object to be scanned at most once during each GC.  This
+    /// currently only affects the MarkSweep plan.
+    const UNIQUE_OBJECT_ENQUEUING: bool = true;
+
     fn support_slot_enqueuing(_tls: VMWorkerThread, _object: ObjectReference) -> bool {
         false
     }