bql: Fix bql_locked status with condvar APIs

QEMU has a per-thread "bql_locked" variable stored in TLS section, showing
whether the current thread is holding the BQL lock.

It's a pretty handy variable.  Function-wise, QEMU have codes trying to
conditionally take bql, relying on the var reflecting the locking status
(e.g. BQL_LOCK_GUARD), or in a GDB debugging session, we could also look at
the variable (in reality, co_tls_bql_locked), to see which thread is
currently holding the bql.

When using that as a debugging facility, sometimes we can observe multiple
threads holding bql at the same time. It's because QEMU's condvar APIs
bypassed the bql_*() API, hence they do not update bql_locked even if they
have released the mutex while waiting.

It can cause confusion if one does "thread apply all p co_tls_bql_locked"
and see multiple threads reporting true.

Fix this by moving the bql status updates into the mutex debug hooks.  Now
the variable should always reflect the reality.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250904223158.1276992-1-peterx@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

include/qemu/main-loop.h

@@ -270,6 +270,24 @@ void rust_bql_mock_lock(void);
  */
 bool bql_locked(void);
 
+/**
+ * mutex_is_bql:
+ *
+ * @mutex: the mutex pointer
+ *
+ * Returns whether the mutex is the BQL.
+ */
+bool mutex_is_bql(QemuMutex *mutex);
+
+/**
+ * bql_update_status:
+ *
+ * @locked: update status on whether the BQL is locked
+ *
+ * NOTE: this should normally only be invoked when the status changed.
+ */
+void bql_update_status(bool locked);
+
 /**
  * bql_block: Allow/deny releasing the BQL
  *

stubs/iothread-lock.c

@@ -34,3 +34,12 @@ void bql_block_unlock(bool increase)
     assert((new_value > bql_unlock_blocked) == increase);
     bql_unlock_blocked = new_value;
 }
+
+bool mutex_is_bql(QemuMutex *mutex)
+{
+    return false;
+}
+
+void bql_update_status(bool locked)
+{
+}

system/cpus.c

@@ -524,6 +524,18 @@ bool qemu_in_vcpu_thread(void)
 
 QEMU_DEFINE_STATIC_CO_TLS(bool, bql_locked)
 
+bool mutex_is_bql(QemuMutex *mutex)
+{
+    return mutex == &bql;
+}
+
+void bql_update_status(bool locked)
+{
+    /* This function should only be used when an update happened.. */
+    assert(bql_locked() != locked);
+    set_bql_locked(locked);
+}
+
 static uint32_t bql_unlock_blocked;
 
 void bql_block_unlock(bool increase)
@@ -564,14 +576,12 @@ void bql_lock_impl(const char *file, int line)
 
     g_assert(!bql_locked());
     bql_lock_fn(&bql, file, line);
-    set_bql_locked(true);
 }
 
 void bql_unlock(void)
 {
     g_assert(bql_locked());
     g_assert(!bql_unlock_blocked);
-    set_bql_locked(false);
     qemu_mutex_unlock(&bql);
 }
 

util/qemu-thread-common.h

@@ -14,6 +14,7 @@
 #define QEMU_THREAD_COMMON_H
 
 #include "qemu/thread.h"
+#include "qemu/main-loop.h"
 #include "trace.h"
 
 static inline void qemu_mutex_post_init(QemuMutex *mutex)
@@ -39,6 +40,9 @@ static inline void qemu_mutex_post_lock(QemuMutex *mutex,
     mutex->line = line;
 #endif
     trace_qemu_mutex_locked(mutex, file, line);
+    if (mutex_is_bql(mutex)) {
+        bql_update_status(true);
+    }
 }
 
 static inline void qemu_mutex_pre_unlock(QemuMutex *mutex,
@@ -49,6 +53,9 @@ static inline void qemu_mutex_pre_unlock(QemuMutex *mutex,
     mutex->line = 0;
 #endif
     trace_qemu_mutex_unlock(mutex, file, line);
+    if (mutex_is_bql(mutex)) {
+        bql_update_status(false);
+    }
 }
 
 #endif