Re: [PATCH v2 4/4] block: Close block exports in two steps

[Kevin Wolf <kwolf@redhat.com>]

Am 14.12.2020 um 18:05 hat Sergio Lopez geschrieben:
> There's a cross-dependency between closing the block exports and
> draining the block layer. The latter needs that we close all export's
> client connections to ensure they won't queue more requests, but the
> exports may have coroutines yielding in the block layer, which implies
> they can't be fully closed until we drain it.

A coroutine that yielded must have some way to be reentered. So I guess
the quesiton becomes why they aren't reentered until drain. We do
process events:

    AIO_WAIT_WHILE(NULL, blk_exp_has_type(type));

So in theory, anything that would finalise the block export closing
should still execute.

What is the difference that drain makes compared to a simple
AIO_WAIT_WHILE, so that coroutine are reentered during drain, but not
during AIO_WAIT_WHILE?

This is an even more interesting question because the NBD server isn't a
block node nor a BdrvChildClass implementation, so it shouldn't even
notice a drain operation.

Kevin

> To break this cross-dependency, this change adds a "bool wait"
> argument to blk_exp_close_all() and blk_exp_close_all_type(), so
> callers can decide whether they want to wait for the exports to be
> fully quiesced, or just return after requesting them to shut down.
> 
> Then, in bdrv_close_all we make two calls, one without waiting to
> close all client connections, and another after draining the block
> layer, this time waiting for the exports to be fully quiesced.
> 
> RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1900505
> Signed-off-by: Sergio Lopez <slp@redhat.com>
> ---
>  block.c                   | 20 +++++++++++++++++++-
>  block/export/export.c     | 10 ++++++----
>  blockdev-nbd.c            |  2 +-
>  include/block/export.h    |  4 ++--
>  qemu-nbd.c                |  2 +-
>  stubs/blk-exp-close-all.c |  2 +-
>  6 files changed, 30 insertions(+), 10 deletions(-)
> 
> diff --git a/block.c b/block.c
> index bc8a66ab6e..41db70ac07 100644
> --- a/block.c
> +++ b/block.c
> @@ -4472,13 +4472,31 @@ static void bdrv_close(BlockDriverState *bs)
>  void bdrv_close_all(void)
>  {
>      assert(job_next(NULL) == NULL);
> -    blk_exp_close_all();
> +
> +    /*
> +     * There's a cross-dependency between closing the block exports and
> +     * draining the block layer. The latter needs that we close all export's
> +     * client connections to ensure they won't queue more requests, but the
> +     * exports may have coroutines yielding in the block layer, which implies
> +     * they can't be fully closed until we drain it.
> +     *
> +     * Make a first call to close all export's client connections, without
> +     * waiting for each export to be fully quiesced.
> +     */
> +    blk_exp_close_all(false);
>  
>      /* Drop references from requests still in flight, such as canceled block
>       * jobs whose AIO context has not been polled yet */
>      bdrv_drain_all();
>  
>      blk_remove_all_bs();
> +
> +    /*
> +     * Make a second call to shut down the exports, this time waiting for them
> +     * to be fully quiesced.
> +     */
> +    blk_exp_close_all(true);
> +
>      blockdev_close_all_bdrv_states();
>  
>      assert(QTAILQ_EMPTY(&all_bdrv_states));
> diff --git a/block/export/export.c b/block/export/export.c
> index bad6f21b1c..0124ebd9f9 100644
> --- a/block/export/export.c
> +++ b/block/export/export.c
> @@ -280,7 +280,7 @@ static bool blk_exp_has_type(BlockExportType type)
>  }
>  
>  /* type == BLOCK_EXPORT_TYPE__MAX for all types */
> -void blk_exp_close_all_type(BlockExportType type)
> +void blk_exp_close_all_type(BlockExportType type, bool wait)
>  {
>      BlockExport *exp, *next;
>  
> @@ -293,12 +293,14 @@ void blk_exp_close_all_type(BlockExportType type)
>          blk_exp_request_shutdown(exp);
>      }
>  
> -    AIO_WAIT_WHILE(NULL, blk_exp_has_type(type));
> +    if (wait) {
> +        AIO_WAIT_WHILE(NULL, blk_exp_has_type(type));
> +    }
>  }
>  
> -void blk_exp_close_all(void)
> +void blk_exp_close_all(bool wait)
>  {
> -    blk_exp_close_all_type(BLOCK_EXPORT_TYPE__MAX);
> +    blk_exp_close_all_type(BLOCK_EXPORT_TYPE__MAX, wait);
>  }
>  
>  void qmp_block_export_add(BlockExportOptions *export, Error **errp)
> diff --git a/blockdev-nbd.c b/blockdev-nbd.c
> index d8443d235b..d71d4da7c2 100644
> --- a/blockdev-nbd.c
> +++ b/blockdev-nbd.c
> @@ -266,7 +266,7 @@ void qmp_nbd_server_stop(Error **errp)
>          return;
>      }
>  
> -    blk_exp_close_all_type(BLOCK_EXPORT_TYPE_NBD);
> +    blk_exp_close_all_type(BLOCK_EXPORT_TYPE_NBD, true);
>  
>      nbd_server_free(nbd_server);
>      nbd_server = NULL;
> diff --git a/include/block/export.h b/include/block/export.h
> index 7feb02e10d..71c25928ce 100644
> --- a/include/block/export.h
> +++ b/include/block/export.h
> @@ -83,7 +83,7 @@ BlockExport *blk_exp_find(const char *id);
>  void blk_exp_ref(BlockExport *exp);
>  void blk_exp_unref(BlockExport *exp);
>  void blk_exp_request_shutdown(BlockExport *exp);
> -void blk_exp_close_all(void);
> -void blk_exp_close_all_type(BlockExportType type);
> +void blk_exp_close_all(bool wait);
> +void blk_exp_close_all_type(BlockExportType type, bool wait);
>  
>  #endif
> diff --git a/qemu-nbd.c b/qemu-nbd.c
> index a7075c5419..928f4466f6 100644
> --- a/qemu-nbd.c
> +++ b/qemu-nbd.c
> @@ -1122,7 +1122,7 @@ int main(int argc, char **argv)
>      do {
>          main_loop_wait(false);
>          if (state == TERMINATE) {
> -            blk_exp_close_all();
> +            blk_exp_close_all(true);
>              state = TERMINATED;
>          }
>      } while (state != TERMINATED);
> diff --git a/stubs/blk-exp-close-all.c b/stubs/blk-exp-close-all.c
> index 1c71316763..ecd0ce611f 100644
> --- a/stubs/blk-exp-close-all.c
> +++ b/stubs/blk-exp-close-all.c
> @@ -2,6 +2,6 @@
>  #include "block/export.h"
>  
>  /* Only used in programs that support block exports (libblockdev.fa) */
> -void blk_exp_close_all(void)
> +void blk_exp_close_all(bool wait)
>  {
>  }
> -- 
> 2.26.2
>