qemu-thread: add QemuEvent

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.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2013-09-25 14:20:59 +08:00
parent cb365646a9
commit c7c4d063f5
5 changed files with 161 additions and 0 deletions

View File

@ -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;
};

View File

@ -17,6 +17,10 @@ struct QemuSemaphore {
HANDLE sema;
};
struct QemuEvent {
HANDLE event;
};
typedef struct QemuThreadData QemuThreadData;
struct QemuThread {
QemuThreadData *data;

View File

@ -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);

View File

@ -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)
{
@ -272,6 +277,117 @@ 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)

View File

@ -227,6 +227,32 @@ 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 *);