Re: [Qemu-block] [PATCH 06/10] aio-posix: remove walking_handlers, protecting AioHandler list with list_lock

[Stefan Hajnoczi]

On Tue, Nov 29, 2016 at 12:47:03PM +0100, Paolo Bonzini wrote:
> @@ -272,22 +275,32 @@ bool aio_prepare(AioContext *ctx)
>  bool aio_pending(AioContext *ctx)
>  {
>      AioHandler *node;
> +    bool result = false;
>  
> -    QLIST_FOREACH(node, &ctx->aio_handlers, node) {
> +    /*
> +     * We have to walk very carefully in case aio_set_fd_handler is
> +     * called while we're walking.
> +     */
> +    qemu_lockcnt_inc(&ctx->list_lock);
> +
> +    QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) {
>          int revents;
>  
>          revents = node->pfd.revents & node->pfd.events;
>          if (revents & (G_IO_IN | G_IO_HUP | G_IO_ERR) && node->io_read &&
>              aio_node_check(ctx, node->is_external)) {
> -            return true;
> +            result = true;
> +            break;
>          }
>          if (revents & (G_IO_OUT | G_IO_ERR) && node->io_write &&
>              aio_node_check(ctx, node->is_external)) {
> -            return true;
> +            result = true;
> +            break;
>          }
>      }
> +    qemu_lockcnt_dec(&ctx->list_lock);
>  
> -    return false;
> +    return result;
>  }
>  
>  bool aio_dispatch(AioContext *ctx)
> @@ -308,13 +321,12 @@ bool aio_dispatch(AioContext *ctx)
>       * We have to walk very carefully in case aio_set_fd_handler is
>       * called while we're walking.
>       */
> -    ctx->walking_handlers++;
> +    qemu_lockcnt_inc(&ctx->list_lock);
>  
> -    QLIST_FOREACH_SAFE(node, &ctx->aio_handlers, node, tmp) {
> +    QLIST_FOREACH_SAFE_RCU(node, &ctx->aio_handlers, node, tmp) {
>          int revents;
>  
> -        revents = node->pfd.revents & node->pfd.events;
> -        node->pfd.revents = 0;
> +        revents = atomic_xchg(&node->pfd.revents, 0) & node->pfd.events;

Why is node->pfd.revents accessed with atomic_*() here and in aio_poll()
but not in aio_pending()?

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