Re: [PATCH V12 05/15] block/export: track IOThread references

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Sat, Aug 15, 2026 at 01:48:19AM +0800, Zhang Chen wrote:
> Track the IOThreads used by a block export and identify the holder
> with the unique BlockExportOptions id.
> 
> Acquire holder-aware references during export creation and release
> them on error or export deletion.  Support both single- and
> multi-iothread exports.
> 
> Signed-off-by: Zhang Chen <zhangckid@gmail.com>
> ---
>  block/export/export.c  | 79 +++++++++++++++++++++++++++++++++++++-----
>  include/block/export.h |  6 ++++
>  2 files changed, 76 insertions(+), 9 deletions(-)
> 
> diff --git a/block/export/export.c b/block/export/export.c
> index b733f269f3..09136cda34 100644
> --- a/block/export/export.c
> +++ b/block/export/export.c
> @@ -15,7 +15,6 @@
>  
>  #include "block/block.h"
>  #include "system/block-backend.h"
> -#include "system/iothread.h"
>  #include "block/export.h"
>  #include "block/fuse.h"
>  #include "block/nbd.h"
> @@ -85,6 +84,8 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
>      AioContext *ctx;
>      AioContext **multithread_ctxs = NULL;
>      size_t multithread_count = 0;
> +    g_autofree IOThread **local_iothreads = NULL;
> +    char *holder_id = NULL;
>      uint64_t perm;
>      int ret;
>  
> @@ -139,7 +140,16 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
>              goto fail;
>          }
>  
> -        new_ctx = iothread_get_aio_context(iothread);
> +        holder_id = export->id;
> +        const IOThreadHolder holder = {
> +            .type = IO_THREAD_HOLDER_KIND_BLOCK_EXPORT,
> +            .u.block_export.export_id = holder_id,
> +        };
> +
> +        new_ctx = iothread_ref_and_get_aio_context(iothread, &holder);
> +        multithread_count = 1;
> +        local_iothreads = g_new0(IOThread *, 1);
> +        local_iothreads[0] = iothread;
>  
>          /* Ignore errors with fixed-iothread=false */
>          set_context_errp = fixed_iothread ? errp : NULL;
> @@ -163,8 +173,15 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
>              return NULL;
>          }
>  
> +        local_iothreads = g_new0(IOThread *, multithread_count);
>          multithread_ctxs = g_new(AioContext *, multithread_count);
>          i = 0;
> +        holder_id = export->id;
> +        const IOThreadHolder holder = {
> +            .type = IO_THREAD_HOLDER_KIND_BLOCK_EXPORT,
> +            .u.block_export.export_id = holder_id,
> +        };
> +
>          for (strList *e = iothread_list; e; e = e->next) {
>              IOThread *iothread = iothread_by_id(e->value);
>  
> @@ -172,7 +189,9 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
>                  error_setg(errp, "iothread \"%s\" not found", e->value);
>                  goto fail;
>              }
> -            multithread_ctxs[i++] = iothread_get_aio_context(iothread);
> +            local_iothreads[i] = iothread;
> +            multithread_ctxs[i++] = iothread_ref_and_get_aio_context(iothread,
> +                                                                     &holder);
>          }
>          assert(i == multithread_count);
>      }
> @@ -225,12 +244,15 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
>      assert(drv->instance_size >= sizeof(BlockExport));
>      exp = g_malloc0(drv->instance_size);
>      *exp = (BlockExport) {
> -        .drv        = drv,
> -        .refcount   = 1,
> -        .user_owned = true,
> -        .id         = g_strdup(export->id),
> -        .ctx        = ctx,
> -        .blk        = blk,
> +        .drv                  = drv,
> +        .refcount             = 1,
> +        .user_owned           = true,
> +        .id                   = g_strdup(export->id),
> +        .ctx                  = ctx,
> +        .blk                  = blk,
> +        .iothreads            = g_steal_pointer(&local_iothreads),
> +        .iothread_count       = multithread_count,
> +        .iothread_holder_id   = g_strdup(holder_id),

Is this field necessary, it always seems to be the same as exp->id?

>      };
>  
>      ret = drv->create(exp, export, multithread_ctxs, multithread_count, errp);
> @@ -250,8 +272,33 @@ fail:
>          blk_unref(blk);
>      }
>      if (exp) {
> +        if (exp->iothreads) {
> +            const IOThreadHolder holder = {
> +                .type = IO_THREAD_HOLDER_KIND_BLOCK_EXPORT,
> +                .u.block_export.export_id = exp->iothread_holder_id,
> +            };
> +            for (size_t j = 0; j < exp->iothread_count; j++) {
> +                if (exp->iothreads[j]) {
> +                    iothread_unref_and_put_aio_context(exp->iothreads[j],
> +                                                       &holder);
> +                }
> +            }
> +            g_free(exp->iothreads);
> +        }
> +        g_free(exp->iothread_holder_id);
>          g_free(exp->id);
>          g_free(exp);
> +    } else if (local_iothreads) {
> +        const IOThreadHolder holder = {
> +            .type = IO_THREAD_HOLDER_KIND_BLOCK_EXPORT,
> +            .u.block_export.export_id = holder_id,
> +        };
> +
> +        for (size_t j = 0; j < multithread_count; j++) {
> +            if (local_iothreads[j]) {
> +                iothread_unref_and_put_aio_context(local_iothreads[j], &holder);
> +            }
> +        }
>      }
>      g_free(multithread_ctxs);
>      return NULL;
> @@ -273,6 +320,20 @@ static void blk_exp_delete_bh(void *opaque)
>      exp->drv->delete(exp);
>      blk_set_dev_ops(exp->blk, NULL, NULL);
>      blk_unref(exp->blk);
> +
> +    if (exp->iothreads) {
> +        const IOThreadHolder holder = {
> +            .type = IO_THREAD_HOLDER_KIND_BLOCK_EXPORT,
> +            .u.block_export.export_id = exp->iothread_holder_id,
> +        };
> +
> +        for (size_t i = 0; i < exp->iothread_count; i++) {
> +            iothread_unref_and_put_aio_context(exp->iothreads[i], &holder);
> +        }
> +        g_free(exp->iothreads);
> +    }
> +
> +    g_free(exp->iothread_holder_id);
>      qapi_event_send_block_export_deleted(exp->id);
>      g_free(exp->id);
>      g_free(exp);

There is quite a bit of code duplication with the loops that call
iothread_ref_and_get_aio_context() or
iothread_unref_and_put_aio_context().

Two helper functions could avoid duplication:

  static void init_iothreads(const char *holder_id,
                             IOThread **iothreads,
			     size_t num_iothreads,
                             AioContext **aio_ctxs);

  static void cleanup_iothreads(const char *holder_id,
                                IOThread **iothreads,
				size_t num_iothreads);

> diff --git a/include/block/export.h b/include/block/export.h
> index ca45da928c..a093dea0b6 100644
> --- a/include/block/export.h
> +++ b/include/block/export.h
> @@ -16,6 +16,7 @@
>  
>  #include "qapi/qapi-types-block-export.h"
>  #include "qemu/queue.h"
> +#include "system/iothread.h"
>  
>  typedef struct BlockExport BlockExport;
>  
> @@ -89,6 +90,11 @@ struct BlockExport {
>  
>      /* List entry for block_exports */
>      QLIST_ENTRY(BlockExport) next;
> +
> +    /* The IOThreads utilized by this specific block export */
> +    IOThread **iothreads;
> +    size_t iothread_count;
> +    char *iothread_holder_id;
>  };
>  
>  BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp);
> -- 
> 2.54.0
> 

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