Re: [Qemu-devel] [RFC v2 2/6] aio: add aio_context_acquire() and aio_context_release()

[Fam Zheng]

On Fri, 01/10 09:45, Stefan Hajnoczi wrote:
> 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 <address@hidden>
> ---
>  async.c             | 18 +++++++++++++++++
>  include/block/aio.h | 18 +++++++++++++++++
>  tests/test-aio.c    | 58 
> +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 94 insertions(+)
> 
> diff --git a/async.c b/async.c
> index 5fb3fa6..6930185 100644
> --- a/async.c
> +++ b/async.c
> @@ -214,6 +214,7 @@ aio_ctx_finalize(GSource     *source)
>      thread_pool_free(ctx->thread_pool);
>      aio_set_event_notifier(ctx, &ctx->notifier, NULL);
>      event_notifier_cleanup(&ctx->notifier);
> +    rfifolock_destroy(&ctx->lock);
>      qemu_mutex_destroy(&ctx->bh_lock);
>      g_array_free(ctx->pollfds, TRUE);
>      timerlistgroup_deinit(&ctx->tlg);
> @@ -250,6 +251,12 @@ static void aio_timerlist_notify(void *opaque)
>      aio_notify(opaque);
>  }
>  
> +static void aio_rfifolock_cb(void *opaque)
> +{
> +    /* Kick owner thread in case they are blocked in aio_poll() */
> +    aio_notify(opaque);
> +}
> +
>  AioContext *aio_context_new(void)
>  {
>      AioContext *ctx;
> @@ -257,6 +264,7 @@ AioContext *aio_context_new(void)
>      ctx->pollfds = g_array_new(FALSE, FALSE, sizeof(GPollFD));
>      ctx->thread_pool = NULL;
>      qemu_mutex_init(&ctx->bh_lock);
> +    rfifolock_init(&ctx->lock, aio_rfifolock_cb, ctx);
>      event_notifier_init(&ctx->notifier, false);
>      aio_set_event_notifier(ctx, &ctx->notifier, 
>                             (EventNotifierHandler *)
> @@ -275,3 +283,13 @@ void aio_context_unref(AioContext *ctx)
>  {
>      g_source_unref(&ctx->source);
>  }
> +
> +void aio_context_acquire(AioContext *ctx)
> +{
> +    rfifolock_lock(&ctx->lock);
> +}
> +
> +void aio_context_release(AioContext *ctx)
> +{
> +    rfifolock_unlock(&ctx->lock);
> +}
> diff --git a/include/block/aio.h b/include/block/aio.h
> index 2efdf41..4aaa5d5 100644
> --- a/include/block/aio.h
> +++ b/include/block/aio.h
> @@ -19,6 +19,7 @@
>  #include "qemu/queue.h"
>  #include "qemu/event_notifier.h"
>  #include "qemu/thread.h"
> +#include "qemu/rfifolock.h"
>  #include "qemu/timer.h"
>  
>  typedef struct BlockDriverAIOCB BlockDriverAIOCB;
> @@ -47,6 +48,9 @@ typedef void IOHandler(void *opaque);
>  struct AioContext {
>      GSource source;
>  
> +    /* Protects all fields from multi-threaded access */
> +    RFifoLock lock;
> +
>      /* The list of registered AIO handlers */
>      QLIST_HEAD(, AioHandler) aio_handlers;
>  
> @@ -104,6 +108,20 @@ 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().
> + *
> + * Note that multiple threads calling aio_poll() means timers, BHs, and
> + * callbacks may be invoked from a different thread than they were registered
> + * from.  Therefore, code must use AioContext acquire/release or use
> + * fine-grained synchronization to protect shared state if other threads will
> + * be accessing it simultaneously.
> + */
> +void aio_context_acquire(AioContext *ctx);
> +
> +/* Reliquinish ownership of the AioContext. */

s/Reliquinish/Relinquish/

Fam

> +void aio_context_release(AioContext *ctx);
> +
>  /**
>   * aio_bh_new: Allocate a new bottom half structure.
>   *
> diff --git a/tests/test-aio.c b/tests/test-aio.c
> index 592721e..d384b0b 100644
> --- a/tests/test-aio.c
> +++ b/tests/test-aio.c
> @@ -112,6 +112,63 @@ 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 };
> @@ -775,6 +832,7 @@ 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);
> -- 
> 1.8.4.2
> 
>