Re: [PATCH V7 02/14] iothread: introduce iothread_ref/unref to track attached devices

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, May 11, 2026 at 10:04:04PM +0800, Zhang Chen wrote:
> Currently, IOThreads do not maintain a record of which devices are
> associated with them. This makes it difficult to monitor the
> workload distribution of IOThreads, especially in complex
> hotplug scenarios involving multiple virtio-blk or virtio-scsi devices.
> 
> This patch introduces a reference counting and tracking mechanism
> within the IOThread object:
> 
> - iothread_ref(): Prepends the device's QOM path to a list.
> - iothread_unref(): Searches for the device path using a custom
>   string comparison (g_strcmp0), releases the associated memory
>   upon a successful match.
> - holders: A GList storing the QOM paths of attached devices
>   for runtime introspection.
> 
> A later commit will add QMP commands to let management applications
> query the attachment status of IOThreads.
> 
> Signed-off-by: Zhang Chen <zhangckid@gmail.com>
> ---
>  include/system/iothread.h |  5 +++
>  iothread.c                | 67 +++++++++++++++++++++++++++++++++++++++
>  qapi/misc.json            | 48 ++++++++++++++++++++++++++++
>  3 files changed, 120 insertions(+)
> 
> diff --git a/include/system/iothread.h b/include/system/iothread.h
> index a1ef7696cb..2871b06edc 100644
> --- a/include/system/iothread.h
> +++ b/include/system/iothread.h
> @@ -50,6 +50,11 @@ struct IOThread {
>      bool stopping;              /* has iothread_stop() been called? */
>      bool running;               /* should iothread_run() continue? */
>      int thread_id;
> +    /*
> +     * The list elements are of type IoThreadHolder, which can
> +     * represent either a QOM path or a block node name.
> +     */
> +    GList *holders;
>  
>      /* AioContext poll parameters */
>      int64_t poll_max_ns;
> diff --git a/iothread.c b/iothread.c
> index 3558535b40..b805e4f97d 100644
> --- a/iothread.c
> +++ b/iothread.c
> @@ -25,6 +25,66 @@
>  #include "qemu/rcu.h"
>  #include "qemu/main-loop.h"
>  
> +/*
> + * Add the @holder path to the iothread's tracking list.
> + * The @holder is a QOM path if it starts with '/', else a block node name.
> + */

It would be cleaner to use IoThreadHolder in the iothread_ref() and
iothread_unref() APIs instead of a string:

  static void iothread_ref(IOThread *iothread, const IoThreadHolder *holder);
  static void iothread_unref(IOThread *iothread, const IoThreadHolder *holder);

That way the caller already uses IO_THREAD_HOLDER_KIND_QOM_OBJECT or
IO_THREAD_HOLDER_KIND_BLOCK_NODE to clearly distinguish QOM paths from
block node names. No string parsing is necessary.

> +static void iothread_ref(IOThread *iothread, const char *holder)
> +{
> +    IoThreadHolder *h = g_new0(IoThreadHolder, 1);

IoThreadHolder vs IOThread. Name it IOThreadHolder for consistency?

> +
> +    assert(holder);
> +
> +    if (holder[0] == '/') {
> +        h->type = IO_THREAD_HOLDER_KIND_QOM_OBJECT;
> +        h->u.qom_object.data = g_strdup(holder);
> +    } else {
> +        h->type = IO_THREAD_HOLDER_KIND_BLOCK_NODE;
> +        h->u.block_node.data = g_strdup(holder);
> +    }
> +
> +    iothread->holders = g_list_prepend(iothread->holders, h);
> +}
> +
> +static int iothread_holder_compare(gconstpointer a, gconstpointer b)
> +{
> +    const IoThreadHolder *holder_node = a;
> +    const char *target_name = b;
> +    const char *current_name;
> +
> +    if (holder_node->type == IO_THREAD_HOLDER_KIND_QOM_OBJECT) {
> +        current_name = holder_node->u.qom_object.data;
> +    } else if (holder_node->type == IO_THREAD_HOLDER_KIND_BLOCK_NODE) {
> +        current_name = holder_node->u.block_node.data;
> +    } else {
> +        /*
> +         * This should not happen. If it does, current_name remains
> +         * NULL and g_strcmp0 will handle it safely.
> +         */
> +        current_name = NULL;
> +    }
> +
> +    return g_strcmp0(current_name, target_name);
> +}
> +
> +/*
> + * This function removes the @holder from the @iothread's tracking list.
> + * The @holder string must match the one used previously in iothread_ref().
> + * It is a programming error to call this with a @holder that is not
> + * currently associated with the @iothread.
> + */
> +static void iothread_unref(IOThread *iothread, const char *holder)
> +{
> +    GList *link = g_list_find_custom(iothread->holders, holder,
> +                                     (GCompareFunc)iothread_holder_compare);
> +
> +    assert(link);
> +
> +    IoThreadHolder *h = (IoThreadHolder *)link->data;
> +    qapi_free_IoThreadHolder(h);
> +    iothread->holders = g_list_delete_link(iothread->holders, link);
> +}
> +
>  static void *iothread_run(void *opaque)
>  {
>      IOThread *iothread = opaque;
> @@ -108,6 +168,9 @@ static void iothread_instance_finalize(Object *obj)
>  
>      iothread_stop(iothread);
>  
> +    /* We don't support finalize without holders */

The double-negative is confusing and the logic seems incorrect: we don't
support finalize while there are holders. A clearer comment would be:

  /* All holders must have called iothread_unref() */

> +    assert(iothread->holders == NULL);
> +
>      /*
>       * Before glib2 2.33.10, there is a glib2 bug that GSource context
>       * pointer may not be cleared even if the context has already been
> @@ -356,6 +419,10 @@ char *iothread_get_id(IOThread *iothread)
>  
>  AioContext *iothread_get_aio_context(IOThread *iothread)
>  {
> +    /* Remove in next patch for build */
> +    iothread_ref(iothread, "tmp");
> +    iothread_unref(iothread, "tmp");
> +
>      return iothread->ctx;
>  }
>  
> diff --git a/qapi/misc.json b/qapi/misc.json
> index c71a5fe657..5fb7dcfcad 100644
> --- a/qapi/misc.json
> +++ b/qapi/misc.json
> @@ -67,6 +67,54 @@
>  ##
>  { 'command': 'query-name', 'returns': 'NameInfo', 'allow-preconfig': true }
>  
> +
> +##
> +# @IoThreadHolderBlockNode:
> +#
> +# @data: Block node name.
> +#
> +# Since: 11.1
> +#
> +##
> +{ 'struct': 'IoThreadHolderBlockNode',
> +  'data': { 'data': 'str' } }
> +
> +##
> +# @IoThreadHolderQomObject:
> +#
> +# @data: Absolute @qom-path.
> +#
> +# Since: 11.1
> +#
> +##
> +{ 'struct': 'IoThreadHolderQomObject',
> +  'data': { 'data': 'str' } }
> +
> +##
> +# @IoThreadHolderKind:
> +#
> +# @block-node: Block node name.
> +# @qom-object: Absolute @qom-path.
> +#
> +# Since: 11.1
> +##
> +{ 'enum': 'IoThreadHolderKind',
> +  'data': [ 'block-node', 'qom-object' ] }
> +
> +##
> +# @IoThreadHolder:
> +#
> +# @type: the kind of I/O thread holder.
> +#
> +# Since: 11.1
> +##
> +{ 'union': 'IoThreadHolder',
> +  'base': { 'type': 'IoThreadHolderKind' },
> +  'discriminator': 'type',
> +  'data': {
> +    'block-node': 'IoThreadHolderBlockNode',
> +    'qom-object': 'IoThreadHolderQomObject' } }
> +
>  ##
>  # @IOThreadInfo:
>  #
> -- 
> 2.49.0
> 

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