Re: [Qemu-devel] [PATCH v5 07/18] qemu-thread: add simple test-and-set spinlock

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

On Wed, May 18, 2016 at 17:09:48 +0200, Paolo Bonzini wrote:
> But honestly I think it would be even better to just use
> __sync_lock_test_and_set in the spinlock implementation and not add this
> to atomics.h.  There's already enough issues with the current subset of
> atomics, I am not really happy to add non-SC read-modify-write
> operations to the mix.

I can drop the two patches that touch atomic.h, and have the
spinlock patch as appended. OK with this?

Thanks,

		Emilio

commit 99c8f1049a1508edc9de30e3cf27898888aa7f68
Author: Guillaume Delbergue <guillaume.delbergue@greensocs.com>
Date:   Sun Oct 18 09:44:02 2015 +0200

    qemu-thread: add simple test-and-set spinlock
    
    Signed-off-by: Guillaume Delbergue <guillaume.delbergue@greensocs.com>
    [Rewritten. - Paolo]
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    [Emilio's additions: use TAS instead of atomic_xchg; emit acquire/release
     barriers; call cpu_relax() while spinning; optimize for uncontended locks by
     acquiring the lock with TAS instead of TATAS; add qemu_spin_locked().]
    Signed-off-by: Emilio G. Cota <cota@braap.org>

diff --git a/include/qemu/thread.h b/include/qemu/thread.h
index bdae6df..2d225ff 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 +63,40 @@ 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)
+{
+    __sync_lock_release(&spin->value);
+}
+
+static inline void qemu_spin_lock(QemuSpin *spin)
+{
+    while (__sync_lock_test_and_set(&spin->value, true)) {
+        while (atomic_read(&spin->value)) {
+            cpu_relax();
+        }
+    }
+}
+
+static inline int qemu_spin_trylock(QemuSpin *spin)
+{
+    if (__sync_lock_test_and_set(&spin->value, true)) {
+        return -EBUSY;
+    }
+    return 0;
+}
+
+static inline bool qemu_spin_locked(QemuSpin *spin)
+{
+    return atomic_read(&spin->value);
+}
+
+static inline void qemu_spin_unlock(QemuSpin *spin)
+{
+    __sync_lock_release(&spin->value);
+}
+
 #endif