qemu/include/qemu/lockable.h

/*
 * Polymorphic locking functions (aka poor man templates)
 *
 * Copyright Red Hat, Inc. 2017, 2018
 *
 * Author: Paolo Bonzini <pbonzini@redhat.com>
 *
 * This work is licensed under the terms of the GNU LGPL, version 2 or later.
 * See the COPYING.LIB file in the top-level directory.
 *
 */

#ifndef QEMU_LOCKABLE_H
#define QEMU_LOCKABLE_H

#include "qemu/coroutine.h"
#include "qemu/thread.h"

typedef void QemuLockUnlockFunc(void *);

struct QemuLockable {
    void *object;
    QemuLockUnlockFunc *lock;
    QemuLockUnlockFunc *unlock;
};

static inline __attribute__((__always_inline__)) QemuLockable *
qemu_make_lockable(void *x, QemuLockable *lockable)
{
    /*
     * We cannot test this in a macro, otherwise we get compiler
     * warnings like "the address of 'm' will always evaluate as 'true'".
     */
    return x ? lockable : NULL;
}

static inline __attribute__((__always_inline__)) QemuLockable *
qemu_null_lockable(void *x)
{
    if (x != NULL) {
        qemu_build_not_reached();
    }
    return NULL;
}

/*
 * In C, compound literals have the lifetime of an automatic variable.
 * In C++ it would be different, but then C++ wouldn't need QemuLockable
 * either...
 */
#define QML_OBJ_(x, name) (&(QemuLockable) {                            \
        .object = (x),                                                  \
        .lock = (QemuLockUnlockFunc *) qemu_ ## name ## _lock,          \
        .unlock = (QemuLockUnlockFunc *) qemu_ ## name ## _unlock       \
    })

/**
 * QEMU_MAKE_LOCKABLE - Make a polymorphic QemuLockable
 *
 * @x: a lock object (currently one of QemuMutex, QemuRecMutex,
 *     CoMutex, QemuSpin).
 *
 * Returns a QemuLockable object that can be passed around
 * to a function that can operate with locks of any kind, or
 * NULL if @x is %NULL.
 *
 * Note the special case for void *, so that we may pass "NULL".
 */
#define QEMU_MAKE_LOCKABLE(x)                                           \
    _Generic((x), QemuLockable *: (x),                                  \
             void *: qemu_null_lockable(x),                             \
             QemuMutex *: qemu_make_lockable(x, QML_OBJ_(x, mutex)),    \
             QemuRecMutex *: qemu_make_lockable(x, QML_OBJ_(x, rec_mutex)), \
             CoMutex *: qemu_make_lockable(x, QML_OBJ_(x, co_mutex)),   \
             QemuSpin *: qemu_make_lockable(x, QML_OBJ_(x, spin)))

/**
 * QEMU_MAKE_LOCKABLE_NONNULL - Make a polymorphic QemuLockable
 *
 * @x: a lock object (currently one of QemuMutex, QemuRecMutex,
 *     CoMutex, QemuSpin).
 *
 * Returns a QemuLockable object that can be passed around
 * to a function that can operate with locks of any kind.
 */
#define QEMU_MAKE_LOCKABLE_NONNULL(x)                           \
    _Generic((x), QemuLockable *: (x),                          \
                  QemuMutex *: QML_OBJ_(x, mutex),              \
                  QemuRecMutex *: QML_OBJ_(x, rec_mutex),       \
                  CoMutex *: QML_OBJ_(x, co_mutex),             \
                  QemuSpin *: QML_OBJ_(x, spin))

static inline void qemu_lockable_lock(QemuLockable *x)
{
    x->lock(x->object);
}

static inline void qemu_lockable_unlock(QemuLockable *x)
{
    x->unlock(x->object);
}

static inline QemuLockable *qemu_lockable_auto_lock(QemuLockable *x)
{
    qemu_lockable_lock(x);
    return x;
}

static inline void qemu_lockable_auto_unlock(QemuLockable *x)
{
    if (x) {
        qemu_lockable_unlock(x);
    }
}

G_DEFINE_AUTOPTR_CLEANUP_FUNC(QemuLockable, qemu_lockable_auto_unlock)

#define WITH_QEMU_LOCK_GUARD_(x, var) \
    for (g_autoptr(QemuLockable) var = \
                qemu_lockable_auto_lock(QEMU_MAKE_LOCKABLE_NONNULL((x))); \
         var; \
         qemu_lockable_auto_unlock(var), var = NULL)

/**
 * WITH_QEMU_LOCK_GUARD - Lock a lock object for scope
 *
 * @x: a lock object (currently one of QemuMutex, CoMutex, QemuSpin).
 *
 * This macro defines a lock scope such that entering the scope takes the lock
 * and leaving the scope releases the lock.  Return statements are allowed
 * within the scope and release the lock.  Break and continue statements leave
 * the scope early and release the lock.
 *
 *   WITH_QEMU_LOCK_GUARD(&mutex) {
 *       ...
 *       if (error) {
 *           return; <-- mutex is automatically unlocked
 *       }
 *
 *       if (early_exit) {
 *           break;  <-- leave this scope early
 *       }
 *       ...
 *   }
 */
#define WITH_QEMU_LOCK_GUARD(x) \
    WITH_QEMU_LOCK_GUARD_((x), glue(qemu_lockable_auto, __COUNTER__))

/**
 * QEMU_LOCK_GUARD - Lock an object until the end of the scope
 *
 * @x: a lock object (currently one of QemuMutex, CoMutex, QemuSpin).
 *
 * This macro takes a lock until the end of the scope.  Return statements
 * release the lock.
 *
 *   ... <-- mutex not locked
 *   QEMU_LOCK_GUARD(&mutex); <-- mutex locked from here onwards
 *   ...
 *   if (error) {
 *       return; <-- mutex is automatically unlocked
 *   }
 */
#define QEMU_LOCK_GUARD(x)                                       \
    g_autoptr(QemuLockable)                                      \
    glue(qemu_lockable_auto, __COUNTER__) G_GNUC_UNUSED =        \
            qemu_lockable_auto_lock(QEMU_MAKE_LOCKABLE((x)))

#endif