Re: [Qemu-devel] [PATCH] coroutine: use AioContext for CoQueue BH

[Paolo Bonzini <pbonzini@redhat.com>]

Il 06/03/2013 15:53, Stefan Hajnoczi ha scritto:
> CoQueue uses a BH to awake coroutines that were made ready to run again
> using qemu_co_queue_next() or qemu_co_queue_restart_all().  The BH
> currently runs in the iothread AioContext and would break coroutines
> that run in a different AioContext.
> 
> This is a slightly tricky problem because the lifetime of the BH exceeds
> that of the CoQueue.  This means coroutines can be awoken after CoQueue
> itself has been freed.  Also, there is no qemu_co_queue_destroy()
> function which we could use to handle freeing resources.
> 
> Introducing qemu_co_queue_destroy() has a ripple effect of requiring us
> to also add qemu_co_mutex_destroy() and qemu_co_rwlock_destroy(), as
> well as updating all callers.  Avoid doing that.
> 
> We also cannot switch from BH to GIdle function because aio_poll() does
> not dispatch GIdle functions.  (GIdle functions make memory management
> slightly easier because they free themselves.)
> 
> Finally, I don't want to move unlock_queue and unlock_bh into
> AioContext.  That would break encapsulation - AioContext isn't supposed
> to know about CoQueue.
> 
> This patch implements a different solution: each qemu_co_queue_next() or
> qemu_co_queue_restart_all() call creates a new BH and list of coroutines
> to wake up.  Callers tend to invoke qemu_co_queue_next() and
> qemu_co_queue_restart_all() occasionally after blocking I/O, so creating
> a new BH for each call shouldn't be massively inefficient.
> 
> Note that this patch does not add an interface for specifying the
> AioContext.  That is left to future patches which will convert CoQueue,
> CoMutex, and CoRwlock to expose AioContext.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  include/block/coroutine.h |  1 +
>  qemu-coroutine-lock.c     | 59 ++++++++++++++++++++++++++++++++---------------
>  2 files changed, 42 insertions(+), 18 deletions(-)
> 
> diff --git a/include/block/coroutine.h b/include/block/coroutine.h
> index c31fae3..a978162 100644
> --- a/include/block/coroutine.h
> +++ b/include/block/coroutine.h
> @@ -104,6 +104,7 @@ bool qemu_in_coroutine(void);
>   */
>  typedef struct CoQueue {
>      QTAILQ_HEAD(, Coroutine) entries;
> +    AioContext *ctx;
>  } CoQueue;
>  
>  /**
> diff --git a/qemu-coroutine-lock.c b/qemu-coroutine-lock.c
> index 97ef01c..ae986b3 100644
> --- a/qemu-coroutine-lock.c
> +++ b/qemu-coroutine-lock.c
> @@ -29,28 +29,34 @@
>  #include "block/aio.h"
>  #include "trace.h"
>  
> -static QTAILQ_HEAD(, Coroutine) unlock_bh_queue =
> -    QTAILQ_HEAD_INITIALIZER(unlock_bh_queue);
> -static QEMUBH* unlock_bh;
> +/* Coroutines are awoken from a BH to allow the current coroutine to complete
> + * its flow of execution.  The BH may run after the CoQueue has been destroyed,
> + * so keep BH data in a separate heap-allocated struct.
> + */
> +typedef struct {
> +    QEMUBH *bh;
> +    QTAILQ_HEAD(, Coroutine) entries;
> +} CoQueueNextData;
>  
>  static void qemu_co_queue_next_bh(void *opaque)
>  {
> +    CoQueueNextData *data = opaque;
>      Coroutine *next;
>  
>      trace_qemu_co_queue_next_bh();
> -    while ((next = QTAILQ_FIRST(&unlock_bh_queue))) {
> -        QTAILQ_REMOVE(&unlock_bh_queue, next, co_queue_next);
> +    while ((next = QTAILQ_FIRST(&data->entries))) {
> +        QTAILQ_REMOVE(&data->entries, next, co_queue_next);
>          qemu_coroutine_enter(next, NULL);
>      }
> +
> +    qemu_bh_delete(data->bh);
> +    g_slice_free(CoQueueNextData, data);
>  }
>  
>  void qemu_co_queue_init(CoQueue *queue)
>  {
>      QTAILQ_INIT(&queue->entries);
> -
> -    if (!unlock_bh) {
> -        unlock_bh = qemu_bh_new(qemu_co_queue_next_bh, NULL);
> -    }
> +    queue->ctx = NULL;

What about adding an accessor for qemu_aio_context and using it?  Then
you can just use aio_bh_new in qemu_co_queue_do_restart.

Paolo

>  }
>  
>  void coroutine_fn qemu_co_queue_wait(CoQueue *queue)
> @@ -69,26 +75,43 @@ void coroutine_fn qemu_co_queue_wait_insert_head(CoQueue *queue)
>      assert(qemu_in_coroutine());
>  }
>  
> -bool qemu_co_queue_next(CoQueue *queue)
> +static bool qemu_co_queue_do_restart(CoQueue *queue, bool single)
>  {
>      Coroutine *next;
> +    CoQueueNextData *data;
> +
> +    if (QTAILQ_EMPTY(&queue->entries)) {
> +        return false;
> +    }
>  
> -    next = QTAILQ_FIRST(&queue->entries);
> -    if (next) {
> +    data = g_slice_new(CoQueueNextData);
> +    if (queue->ctx) {
> +        data->bh = aio_bh_new(queue->ctx, qemu_co_queue_next_bh, data);
> +    } else {
> +        data->bh = qemu_bh_new(qemu_co_queue_next_bh, data);
> +    }
> +    QTAILQ_INIT(&data->entries);
> +    qemu_bh_schedule(data->bh);
> +
> +    while ((next = QTAILQ_FIRST(&queue->entries)) != NULL) {
>          QTAILQ_REMOVE(&queue->entries, next, co_queue_next);
> -        QTAILQ_INSERT_TAIL(&unlock_bh_queue, next, co_queue_next);
> +        QTAILQ_INSERT_TAIL(&data->entries, next, co_queue_next);
>          trace_qemu_co_queue_next(next);
> -        qemu_bh_schedule(unlock_bh);
> +        if (single) {
> +            break;
> +        }
>      }
> +    return true;
> +}
>  
> -    return (next != NULL);
> +bool qemu_co_queue_next(CoQueue *queue)
> +{
> +    return qemu_co_queue_do_restart(queue, true);
>  }
>  
>  void qemu_co_queue_restart_all(CoQueue *queue)
>  {
> -    while (qemu_co_queue_next(queue)) {
> -        /* Do nothing */
> -    }
> +    qemu_co_queue_do_restart(queue, false);
>  }
>  
>  bool qemu_co_queue_empty(CoQueue *queue)
>