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From: "Emilio G. Cota" <cota@braap.org>
Date: Wed, 20 Apr 2016 13:35:55 -0400
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References: <1461107270-19234-6-git-send-email-cota@braap.org>
Subject: [Qemu-devel] [UPDATED v3 05/11] qemu-thread: add simple
 test-and-set spinlock
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To: QEMU Developers <qemu-devel@nongnu.org>, MTTCG Devel <mttcg@listserver.greensocs.com>
Cc: =?UTF-8?q?Alex=20Benn=C3=A9e?= <alex.bennee@linaro.org>, Paolo Bonzini <pbonzini@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, Richard Henderson <rth@twiddle.net>, Peter Maydell <peter.maydell@linaro.org>, Sergey Fedorov <serge.fdrv@gmail.com>

From: Guillaume Delbergue <guillaume.delbergue@greensocs.com>

Signed-off-by: Guillaume Delbergue <guillaume.delbergue@greensocs.com>
[Rewritten. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[Emilio's additions: call cpu_relax() while spinning; optimize for
 uncontended locks by acquiring the lock with test-and-set instead of
 test-and-test-and-set.]
Signed-off-by: Emilio G. Cota <cota@braap.org>
---
 include/qemu/thread.h | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/include/qemu/thread.h b/include/qemu/thread.h
index bdae6df..a216941 100644
--- a/include/qemu/thread.h
+++ b/include/qemu/thread.h
@@ -1,6 +1,9 @@
 #ifndef __QEMU_THREAD_H
 #define __QEMU_THREAD_H 1
 
+#include <errno.h>
+#include "qemu/processor.h"
+#include "qemu/atomic.h"
 
 typedef struct QemuMutex QemuMutex;
 typedef struct QemuCond QemuCond;
@@ -60,4 +63,35 @@ struct Notifier;
 void qemu_thread_atexit_add(struct Notifier *notifier);
 void qemu_thread_atexit_remove(struct Notifier *notifier);
 
+typedef struct QemuSpin {
+    int value;
+} QemuSpin;
+
+static inline void qemu_spin_init(QemuSpin *spin)
+{
+    spin->value = 0;
+}
+
+static inline void qemu_spin_lock(QemuSpin *spin)
+{
+    while (atomic_xchg(&spin->value, true)) {
+        while (atomic_read(&spin->value)) {
+            cpu_relax();
+        }
+    }
+}
+
+static inline int qemu_spin_trylock(QemuSpin *spin)
+{
+    if (atomic_xchg(&spin->value, true)) {
+        return -EBUSY;
+    }
+    return 0;
+}
+
+static inline void qemu_spin_unlock(QemuSpin *spin)
+{
+    atomic_mb_set(&spin->value, 0);
+}
+
 #endif
-- 
2.5.0