[Qemu-devel] [PATCH 07/30] qemu-thread: add QemuEvent

[Paolo Bonzini <pbonzini@redhat.com>]

This emulates Win32 manual-reset events using futexes or conditional
variables.  Typical ways to use them are with multi-producer,
single-consumer data structures, to test for a complex condition whose
elements come from different threads:

    for (;;) {
        qemu_event_reset(ev);
        ... test complex condition ...
        if (condition is true) {
            break;
        }
        qemu_event_wait(ev);
    }

Or more efficiently (but with some duplication):

    ... evaluate condition ...
    while (!condition) {
        qemu_event_reset(ev);
        ... evaluate condition ...
        if (!condition) {
            qemu_event_wait(ev);
            ... evaluate condition ...
        }
    }

QemuEvent provides a very fast userspace path in the common case when
no other thread is waiting, or the event is not changing state.  It
is used to report RCU quiescent states to the thread calling
synchronize_rcu (the latter being the single consumer), and to report
call_rcu invocations to the thread that receives them.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 include/qemu/thread-posix.h |   8 +++
 include/qemu/thread-win32.h |   4 ++
 include/qemu/thread.h       |   7 +++
 util/qemu-thread-posix.c    | 116 ++++++++++++++++++++++++++++++++++++++++++++
 util/qemu-thread-win32.c    |  26 ++++++++++
 5 files changed, 161 insertions(+)

diff --git a/include/qemu/thread-posix.h b/include/qemu/thread-posix.h
index 0f30dcc..916b2a7 100644
--- a/include/qemu/thread-posix.h
+++ b/include/qemu/thread-posix.h
@@ -21,6 +21,14 @@ struct QemuSemaphore {
 #endif
 };
 
+struct QemuEvent {
+#ifndef __linux__
+    pthread_mutex_t lock;
+    pthread_cond_t cond;
+#endif
+    unsigned value;
+};
+
 struct QemuThread {
     pthread_t thread;
 };
diff --git a/include/qemu/thread-win32.h b/include/qemu/thread-win32.h
index 13adb95..3d58081 100644
--- a/include/qemu/thread-win32.h
+++ b/include/qemu/thread-win32.h
@@ -17,6 +17,10 @@ struct QemuSemaphore {
     HANDLE sema;
 };
 
+struct QemuEvent {
+    HANDLE event;
+};
+
 typedef struct QemuThreadData QemuThreadData;
 struct QemuThread {
     QemuThreadData *data;
diff --git a/include/qemu/thread.h b/include/qemu/thread.h
index c02404b..3e32c65 100644
--- a/include/qemu/thread.h
+++ b/include/qemu/thread.h
@@ -7,6 +7,7 @@
 typedef struct QemuMutex QemuMutex;
 typedef struct QemuCond QemuCond;
 typedef struct QemuSemaphore QemuSemaphore;
+typedef struct QemuEvent QemuEvent;
 typedef struct QemuThread QemuThread;
 
 #ifdef _WIN32
@@ -45,6 +46,12 @@ void qemu_sem_wait(QemuSemaphore *sem);
 int qemu_sem_timedwait(QemuSemaphore *sem, int ms);
 void qemu_sem_destroy(QemuSemaphore *sem);
 
+void qemu_event_init(QemuEvent *ev, bool init);
+void qemu_event_set(QemuEvent *ev);
+void qemu_event_reset(QemuEvent *ev);
+void qemu_event_wait(QemuEvent *ev);
+void qemu_event_destroy(QemuEvent *ev);
+
 void qemu_thread_create(QemuThread *thread,
                         void *(*start_routine)(void *),
                         void *arg, int mode);
diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index 4489abf..8178f9b 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -20,7 +20,12 @@
 #include <limits.h>
 #include <unistd.h>
 #include <sys/time.h>
+#ifdef __linux__
+#include <sys/syscall.h>
+#include <linux/futex.h>
+#endif
 #include "qemu/thread.h"
+#include "qemu/atomic.h"
 
 static void error_exit(int err, const char *msg)
 {
@@ -268,6 +273,117 @@ void qemu_sem_wait(QemuSemaphore *sem)
 #endif
 }
 
+#ifdef __linux__
+#define futex(...)              syscall(__NR_futex, __VA_ARGS__)
+
+static inline void futex_wake(QemuEvent *ev, int n)
+{
+    futex(ev, FUTEX_WAKE, n, NULL, NULL, 0);
+}
+
+static inline void futex_wait(QemuEvent *ev, unsigned val)
+{
+    futex(ev, FUTEX_WAIT, (int) val, NULL, NULL, 0);
+}
+#else
+static inline void futex_wake(QemuEvent *ev, int n)
+{
+    if (n == 1) {
+        pthread_cond_signal(&ev->cond);
+    } else {
+        pthread_cond_broadcast(&ev->cond);
+    }
+}
+
+static inline void futex_wait(QemuEvent *ev, unsigned val)
+{
+    pthread_mutex_lock(&ev->lock);
+    if (ev->value == val) {
+        pthread_cond_wait(&ev->cond, &ev->lock);
+    }
+    pthread_mutex_unlock(&ev->lock);
+}
+#endif
+
+/* Valid transitions:
+ * - free->set, when setting the event
+ * - busy->set, when setting the event, followed by futex_wake
+ * - set->free, when resetting the event
+ * - free->busy, when waiting
+ *
+ * set->busy does not happen (it can be observed from the outside but
+ * it really is set->free->busy).
+ *
+ * busy->free provably cannot happen; to enforce it, the set->free transition
+ * is done with an OR, which becomes a no-op if the event has concurrently
+ * transitioned to free or busy.
+ */
+
+#define EV_SET         0
+#define EV_FREE        1
+#define EV_BUSY       -1
+
+void qemu_event_init(QemuEvent *ev, bool init)
+{
+#ifndef __linux__
+    pthread_mutex_init(&ev->lock, NULL);
+    pthread_cond_init(&ev->cond, NULL);
+#endif
+
+    ev->value = (init ? EV_SET : EV_FREE);
+}
+
+void qemu_event_destroy(QemuEvent *ev)
+{
+#ifndef __linux__
+    pthread_mutex_destroy(&ev->lock);
+    pthread_cond_destroy(&ev->cond);
+#endif
+}
+
+void qemu_event_set(QemuEvent *ev)
+{
+    if (atomic_mb_read(&ev->value) != EV_SET) {
+        if (atomic_xchg(&ev->value, EV_SET) == EV_BUSY) {
+            /* There were waiters, wake them up.  */
+            futex_wake(ev, INT_MAX);
+        }
+    }
+}
+
+void qemu_event_reset(QemuEvent *ev)
+{
+    if (atomic_mb_read(&ev->value) == EV_SET) {
+        /*
+         * If there was a concurrent reset (or even reset+wait),
+         * do nothing.  Otherwise change EV_SET->EV_FREE.
+         */
+        atomic_or(&ev->value, EV_FREE);
+    }
+}
+
+void qemu_event_wait(QemuEvent *ev)
+{
+    unsigned value;
+
+    value = atomic_mb_read(&ev->value);
+    if (value != EV_SET) {
+        if (value == EV_FREE) {
+            /*
+             * Leave the event reset and tell qemu_event_set that there
+             * are waiters.  No need to retry, because there cannot be
+             * a concurent busy->free transition.  After the CAS, the
+             * event will be either set or busy.
+             */
+            if (atomic_cmpxchg(&ev->value, EV_FREE, EV_BUSY) == EV_SET) {
+                return;
+            }
+        }
+        futex_wait(ev, EV_BUSY);
+    }
+}
+
+
 void qemu_thread_create(QemuThread *thread,
                        void *(*start_routine)(void*),
                        void *arg, int mode)
diff --git a/util/qemu-thread-win32.c b/util/qemu-thread-win32.c
index f75e404..de49f1e 100644
--- a/util/qemu-thread-win32.c
+++ b/util/qemu-thread-win32.c
@@ -244,6 +244,32 @@ void qemu_sem_wait(QemuSemaphore *sem)
     }
 }
 
+void qemu_event_init(QemuEvent *ev, bool init)
+{
+    /* Manual reset.  */
+    ev->event = CreateEvent(NULL, TRUE, init, NULL);
+}
+
+void qemu_event_destroy(QemuEvent *ev)
+{
+    CloseHandle(ev->event);
+}
+
+void qemu_event_set(QemuEvent *ev)
+{
+    SetEvent(ev->event);
+}
+
+void qemu_event_reset(QemuEvent *ev)
+{
+    ResetEvent(ev->event);
+}
+
+void qemu_event_wait(QemuEvent *ev)
+{
+    WaitForSingleObject(ev->event, INFINITE);
+}
+
 struct QemuThreadData {
     /* Passed to win32_start_routine.  */
     void             *(*start_routine)(void *);
-- 
1.8.1.4