[Qemu-devel] [RFC v3 03/56] cpu: introduce cpu_mutex_lock/unlock

["Emilio G. Cota" <cota@braap.org>]

The few direct users of &cpu->lock will be converted soon.

Cc: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Cc: Richard Henderson <rth@twiddle.net>
Signed-off-by: Emilio G. Cota <cota@braap.org>
---
 include/qom/cpu.h   | 26 ++++++++++++++++++++++++
 cpus.c              | 48 +++++++++++++++++++++++++++++++++++++++++++--
 stubs/cpu-lock.c    | 15 ++++++++++++++
 stubs/Makefile.objs |  1 +
 4 files changed, 88 insertions(+), 2 deletions(-)
 create mode 100644 stubs/cpu-lock.c

diff --git a/include/qom/cpu.h b/include/qom/cpu.h
index b813ca28fa..1292e7aa33 100644
--- a/include/qom/cpu.h
+++ b/include/qom/cpu.h
@@ -452,6 +452,32 @@ extern struct CPUTailQ cpus;
 
 extern __thread CPUState *current_cpu;
 
+/**
+ * cpu_mutex_lock - lock a CPU's mutex
+ * @cpu: the CPU whose mutex is to be locked
+ *
+ * To avoid deadlock, a CPU's mutex must be acquired after the BQL.
+ */
+#define cpu_mutex_lock(cpu)                             \
+    cpu_mutex_lock_impl(cpu, __FILE__, __LINE__)
+void cpu_mutex_lock_impl(CPUState *cpu, const char *file, int line);
+
+/**
+ * cpu_mutex_unlock - unlock a CPU's mutex
+ * @cpu: the CPU whose mutex is to be unlocked
+ */
+#define cpu_mutex_unlock(cpu)                           \
+    cpu_mutex_unlock_impl(cpu, __FILE__, __LINE__)
+void cpu_mutex_unlock_impl(CPUState *cpu, const char *file, int line);
+
+/**
+ * cpu_mutex_locked - check whether a CPU's mutex is locked
+ * @cpu: the CPU of interest
+ *
+ * Returns true if the calling thread is currently holding the CPU's mutex.
+ */
+bool cpu_mutex_locked(const CPUState *cpu);
+
 static inline void cpu_tb_jmp_cache_clear(CPUState *cpu)
 {
     unsigned int i;
diff --git a/cpus.c b/cpus.c
index b2a9698dc0..a190651653 100644
--- a/cpus.c
+++ b/cpus.c
@@ -83,6 +83,47 @@ static unsigned int throttle_percentage;
 #define CPU_THROTTLE_PCT_MAX 99
 #define CPU_THROTTLE_TIMESLICE_NS 10000000
 
+/* XXX: is this really the max number of CPUs? */
+#define CPU_LOCK_BITMAP_SIZE 2048
+
+/*
+ * Note: we index the bitmap with cpu->cpu_index + 1 so that the logic
+ * also works during early CPU initialization, when cpu->cpu_index is set to
+ * UNASSIGNED_CPU_INDEX == -1.
+ */
+static __thread DECLARE_BITMAP(cpu_lock_bitmap, CPU_LOCK_BITMAP_SIZE);
+
+static bool no_cpu_mutex_locked(void)
+{
+    return bitmap_empty(cpu_lock_bitmap, CPU_LOCK_BITMAP_SIZE);
+}
+
+void cpu_mutex_lock_impl(CPUState *cpu, const char *file, int line)
+{
+/* coverity gets confused by the indirect function call */
+#ifdef __COVERITY__
+    qemu_mutex_lock_impl(&cpu->lock, file, line);
+#else
+    QemuMutexLockFunc f = atomic_read(&qemu_mutex_lock_func);
+
+    g_assert(!cpu_mutex_locked(cpu));
+    set_bit(cpu->cpu_index + 1, cpu_lock_bitmap);
+    f(&cpu->lock, file, line);
+#endif
+}
+
+void cpu_mutex_unlock_impl(CPUState *cpu, const char *file, int line)
+{
+    g_assert(cpu_mutex_locked(cpu));
+    qemu_mutex_unlock_impl(&cpu->lock, file, line);
+    clear_bit(cpu->cpu_index + 1, cpu_lock_bitmap);
+}
+
+bool cpu_mutex_locked(const CPUState *cpu)
+{
+    return test_bit(cpu->cpu_index + 1, cpu_lock_bitmap);
+}
+
 bool cpu_is_stopped(CPUState *cpu)
 {
     return cpu->stopped || !runstate_is_running();
@@ -92,9 +133,9 @@ static inline bool cpu_work_list_empty(CPUState *cpu)
 {
     bool ret;
 
-    qemu_mutex_lock(&cpu->lock);
+    cpu_mutex_lock(cpu);
     ret = QSIMPLEQ_EMPTY(&cpu->work_list);
-    qemu_mutex_unlock(&cpu->lock);
+    cpu_mutex_unlock(cpu);
     return ret;
 }
 
@@ -1843,6 +1884,9 @@ void qemu_mutex_lock_iothread_impl(const char *file, int line)
 {
     QemuMutexLockFunc bql_lock = atomic_read(&qemu_bql_mutex_lock_func);
 
+    /* prevent deadlock with CPU mutex */
+    g_assert(no_cpu_mutex_locked());
+
     g_assert(!qemu_mutex_iothread_locked());
     bql_lock(&qemu_global_mutex, file, line);
     iothread_locked = true;
diff --git a/stubs/cpu-lock.c b/stubs/cpu-lock.c
new file mode 100644
index 0000000000..bc54f00b78
--- /dev/null
+++ b/stubs/cpu-lock.c
@@ -0,0 +1,15 @@
+#include "qemu/osdep.h"
+#include "qom/cpu.h"
+
+void cpu_mutex_lock_impl(CPUState *cpu, const char *file, int line)
+{
+}
+
+void cpu_mutex_unlock_impl(CPUState *cpu, const char *file, int line)
+{
+}
+
+bool cpu_mutex_locked(const CPUState *cpu)
+{
+    return true;
+}
diff --git a/stubs/Makefile.objs b/stubs/Makefile.objs
index 53d3f32cb2..fbcdc0256d 100644
--- a/stubs/Makefile.objs
+++ b/stubs/Makefile.objs
@@ -8,6 +8,7 @@ stub-obj-y += blockdev-close-all-bdrv-states.o
 stub-obj-y += clock-warp.o
 stub-obj-y += cpu-get-clock.o
 stub-obj-y += cpu-get-icount.o
+stub-obj-y += cpu-lock.o
 stub-obj-y += dump.o
 stub-obj-y += error-printf.o
 stub-obj-y += fdset.o
-- 
2.17.1