Re: [Qemu-devel] [RFC] aio: add aio_context_acquire() and aio_context_release()

[Paolo Bonzini <pbonzini@redhat.com>]

Il 27/08/2013 16:39, Stefan Hajnoczi ha scritto:
> It can be useful to run an AioContext from a thread which normally does
> not "own" the AioContext.  For example, request draining can be
> implemented by acquiring the AioContext and looping aio_poll() until all
> requests have been completed.
> 
> The following pattern should work:
> 
>   /* Event loop thread */
>   while (running) {
>       aio_context_acquire(ctx);
>       aio_poll(ctx, true);
>       aio_context_release(ctx);
>   }
> 
>   /* Another thread */
>   aio_context_acquire(ctx);
>   bdrv_read(bs, 0x1000, buf, 1);
>   aio_context_release(ctx);
> 
> This patch implements aio_context_acquire() and aio_context_release().
> Note that existing aio_poll() callers do not need to worry about
> acquiring and releasing - it is only needed when multiple threads will
> call aio_poll() on the same AioContext.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

Really, really nice!   The "kick owner thread" technique is a very
interesting way to avoid dropping the lock around aio_poll's ppoll
system call.

On top of this, I think it would be useful to make aio_context_acquire
support recursive acquisition by returning a bool if the current thread
is already the owner.  Recursive acquisition != recursive locking! :)
In fact, acquisition and releasing could be done directly by the
synchronous block I/O functions perhaps?

One comment: ctx->owner is really "ctx->owned", if you replace the
argument of qemu_thread_is_self(ctx->owner) with &ctx->owner_thread.  It
is probably a bit clearer that way.

Paolo


> ---
> I previously sent patches that implement bdrv_drain_all() by stopping dataplane
> threads.  AioContext acquire()/release() is a more general solution than
> temporarily stopping dataplane threads.  This solution is less hacky and also
> supported by other event loops like GMainContext.
> 
> No need to commit this patch yet, I still want to build things on top of it
> before submitting a final version.
> 
>  async.c             | 27 +++++++++++++++++++++++++
>  include/block/aio.h | 13 ++++++++++++
>  tests/test-aio.c    | 58 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 98 insertions(+)
> 
> diff --git a/include/block/aio.h b/include/block/aio.h
> index 5743bf1..9035e87 100644
> --- a/include/block/aio.h
> +++ b/include/block/aio.h
> @@ -45,6 +45,11 @@ typedef void IOHandler(void *opaque);
>  typedef struct AioContext {
>      GSource source;
>  
> +    QemuMutex acquire_lock;
> +    QemuCond acquire_cond;
> +    QemuThread owner_thread;
> +    QemuThread *owner;
> +
>      /* The list of registered AIO handlers */
>      QLIST_HEAD(, AioHandler) aio_handlers;
>  
> @@ -99,6 +104,14 @@ void aio_context_ref(AioContext *ctx);
>   */
>  void aio_context_unref(AioContext *ctx);
>  
> +/* Take ownership of the AioContext.  If the AioContext will be shared between
> + * threads, a thread must have ownership when calling aio_poll().
> + */
> +void aio_context_acquire(AioContext *ctx);
> +
> +/* Reliquinish ownership of the AioContext. */
> +void aio_context_release(AioContext *ctx);
> +
>  /**
>   * aio_bh_new: Allocate a new bottom half structure.
>   *
> diff --git a/async.c b/async.c
> index 9791d8e..9fec07c 100644
> --- a/async.c
> +++ b/async.c
> @@ -203,6 +203,8 @@ aio_ctx_finalize(GSource     *source)
>      thread_pool_free(ctx->thread_pool);
>      aio_set_event_notifier(ctx, &ctx->notifier, NULL);
>      event_notifier_cleanup(&ctx->notifier);
> +    qemu_cond_destroy(&ctx->acquire_cond);
> +    qemu_mutex_destroy(&ctx->acquire_lock);
>      qemu_mutex_destroy(&ctx->bh_lock);
>      g_array_free(ctx->pollfds, TRUE);
>  }
> @@ -240,6 +242,9 @@ AioContext *aio_context_new(void)
>      ctx->pollfds = g_array_new(FALSE, FALSE, sizeof(GPollFD));
>      ctx->thread_pool = NULL;
>      qemu_mutex_init(&ctx->bh_lock);
> +    qemu_mutex_init(&ctx->acquire_lock);
> +    qemu_cond_init(&ctx->acquire_cond);
> +    ctx->owner = NULL;
>      event_notifier_init(&ctx->notifier, false);
>      aio_set_event_notifier(ctx, &ctx->notifier, 
>                             (EventNotifierHandler *)
> @@ -257,3 +262,25 @@ void aio_context_unref(AioContext *ctx)
>  {
>      g_source_unref(&ctx->source);
>  }
> +
> +void aio_context_acquire(AioContext *ctx)
> +{
> +    qemu_mutex_lock(&ctx->acquire_lock);
> +    while (ctx->owner) {
> +        assert(!qemu_thread_is_self(ctx->owner));
> +        aio_notify(ctx); /* kick current owner */
> +        qemu_cond_wait(&ctx->acquire_cond, &ctx->acquire_lock);
> +    }
> +    qemu_thread_get_self(&ctx->owner_thread);
> +    ctx->owner = &ctx->owner_thread;
> +    qemu_mutex_unlock(&ctx->acquire_lock);
> +}
> +
> +void aio_context_release(AioContext *ctx)
> +{
> +    qemu_mutex_lock(&ctx->acquire_lock);
> +    assert(ctx->owner && qemu_thread_is_self(ctx->owner));
> +    ctx->owner = NULL;
> +    qemu_cond_signal(&ctx->acquire_cond);
> +    qemu_mutex_unlock(&ctx->acquire_lock);
> +}
> diff --git a/tests/test-aio.c b/tests/test-aio.c
> index 1ab5637..324c099 100644
> --- a/tests/test-aio.c
> +++ b/tests/test-aio.c
> @@ -88,6 +88,63 @@ static void test_notify(void)
>      g_assert(!aio_poll(ctx, false));
>  }
>  
> +typedef struct {
> +    QemuMutex start_lock;
> +    bool thread_acquired;
> +} AcquireTestData;
> +
> +static void *test_acquire_thread(void *opaque)
> +{
> +    AcquireTestData *data = opaque;
> +
> +    /* Wait for other thread to let us start */
> +    qemu_mutex_lock(&data->start_lock);
> +    qemu_mutex_unlock(&data->start_lock);
> +
> +    aio_context_acquire(ctx);
> +    aio_context_release(ctx);
> +
> +    data->thread_acquired = true; /* success, we got here */
> +
> +    return NULL;
> +}
> +
> +static void dummy_notifier_read(EventNotifier *unused)
> +{
> +    g_assert(false); /* should never be invoked */
> +}
> +
> +static void test_acquire(void)
> +{
> +    QemuThread thread;
> +    EventNotifier notifier;
> +    AcquireTestData data;
> +
> +    /* Dummy event notifier ensures aio_poll() will block */
> +    event_notifier_init(&notifier, false);
> +    aio_set_event_notifier(ctx, &notifier, dummy_notifier_read);
> +    g_assert(!aio_poll(ctx, false)); /* consume aio_notify() */
> +
> +    qemu_mutex_init(&data.start_lock);
> +    qemu_mutex_lock(&data.start_lock);
> +    data.thread_acquired = false;
> +
> +    qemu_thread_create(&thread, test_acquire_thread,
> +                       &data, QEMU_THREAD_JOINABLE);
> +
> +    /* Block in aio_poll(), let other thread kick us and acquire context */
> +    aio_context_acquire(ctx);
> +    qemu_mutex_unlock(&data.start_lock); /* let the thread run */
> +    g_assert(!aio_poll(ctx, true));
> +    aio_context_release(ctx);
> +
> +    qemu_thread_join(&thread);
> +    aio_set_event_notifier(ctx, &notifier, NULL);
> +    event_notifier_cleanup(&notifier);
> +
> +    g_assert(data.thread_acquired);
> +}
> +
>  static void test_bh_schedule(void)
>  {
>      BHTestData data = { .n = 0 };
> @@ -639,6 +696,7 @@ int main(int argc, char **argv)
>  
>      g_test_init(&argc, &argv, NULL);
>      g_test_add_func("/aio/notify",                  test_notify);
> +    g_test_add_func("/aio/acquire",                 test_acquire);
>      g_test_add_func("/aio/bh/schedule",             test_bh_schedule);
>      g_test_add_func("/aio/bh/schedule10",           test_bh_schedule10);
>      g_test_add_func("/aio/bh/cancel",               test_bh_cancel);
>