3 files changed, 45 insertions, 0 deletions

diff --git a/include/qemu/main-loop.h b/include/qemu/main-loop.h
index 5764db157c..646306c272 100644
--- a/include/qemu/main-loop.h
+++ b/include/qemu/main-loop.h
@@ -263,6 +263,21 @@ AioContext *iohandler_get_aio_context(void);
 bool bql_locked(void);
 
 /**
+ * bql_block: Allow/deny releasing the BQL
+ *
+ * The Big QEMU Lock (BQL) is used to provide interior mutability to
+ * Rust code, but this only works if other threads cannot run while
+ * the Rust code has an active borrow.  This is because C code in
+ * other threads could come in and mutate data under the Rust code's
+ * feet.
+ *
+ * @increase: Whether to increase or decrease the blocking counter.
+ *            Releasing the BQL while the counter is nonzero triggers
+ *            an assertion failure.
+ */
+void bql_block_unlock(bool increase);
+
+/**
  * qemu_in_main_thread: return whether it's possible to safely access
  * the global state of the block layer.
  *
diff --git a/stubs/iothread-lock.c b/stubs/iothread-lock.c
index d7890e5581..5467659895 100644
--- a/stubs/iothread-lock.c
+++ b/stubs/iothread-lock.c
@@ -1,6 +1,8 @@
 #include "qemu/osdep.h"
 #include "qemu/main-loop.h"
 
+static uint32_t bql_unlock_blocked;
+
 bool bql_locked(void)
 {
     return false;
@@ -12,4 +14,17 @@ void bql_lock_impl(const char *file, int line)
 
 void bql_unlock(void)
 {
+    assert(!bql_unlock_blocked);
+}
+
+void bql_block_unlock(bool increase)
+{
+    uint32_t new_value;
+
+    assert(bql_locked());
+
+    /* check for overflow! */
+    new_value = bql_unlock_blocked + increase - !increase;
+    assert((new_value > bql_unlock_blocked) == increase);
+    bql_unlock_blocked = new_value;
 }
diff --git a/system/cpus.c b/system/cpus.c
index 1c818ff682..ba633c7688 100644
--- a/system/cpus.c
+++ b/system/cpus.c
@@ -514,6 +514,20 @@ bool qemu_in_vcpu_thread(void)
 
 QEMU_DEFINE_STATIC_CO_TLS(bool, bql_locked)
 
+static uint32_t bql_unlock_blocked;
+
+void bql_block_unlock(bool increase)
+{
+    uint32_t new_value;
+
+    assert(bql_locked());
+
+    /* check for overflow! */
+    new_value = bql_unlock_blocked + increase - !increase;
+    assert((new_value > bql_unlock_blocked) == increase);
+    bql_unlock_blocked = new_value;
+}
+
 bool bql_locked(void)
 {
     return get_bql_locked();
@@ -540,6 +554,7 @@ void bql_lock_impl(const char *file, int line)
 void bql_unlock(void)
 {
     g_assert(bql_locked());
+    g_assert(!bql_unlock_blocked);
     set_bql_locked(false);
     qemu_mutex_unlock(&bql);
 }