Re: [Qemu-devel] [PATCH 1/8] block-migration: acquire AioContext as necessary

[Fam Zheng <famz@redhat.com>]

On Sun, 02/14 18:17, Paolo Bonzini wrote:
> This is needed because dataplane will run during block migration as well.
> 
> The block device migration code is quite liberal in taking the iothread
> mutex.  For simplicity, keep it the same way, even though one could
> actually choose between the BQL (for regular BlockDriverStates) and
> the AioContext (for dataplane BlockDriverStates).  When the block layer
> is made fully thread safe, aio_context_acquire shall go away altogether.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  migration/block.c | 61 ++++++++++++++++++++++++++++++++++++++++++++-----------
>  1 file changed, 49 insertions(+), 12 deletions(-)
> 
> diff --git a/migration/block.c b/migration/block.c
> index a444058..6dd2327 100644
> --- a/migration/block.c
> +++ b/migration/block.c
> @@ -60,9 +60,15 @@ typedef struct BlkMigDevState {
>      int64_t cur_sector;
>      int64_t cur_dirty;
>  
> -    /* Protected by block migration lock.  */
> +    /* Data in the aio_bitmap is protected by block migration lock.
> +     * Allocation and free happen during setup and cleanup respectively.
> +     */
>      unsigned long *aio_bitmap;
> +
> +    /* Protected by block migration lock.  */
>      int64_t completed_sectors;
> +
> +    /* Protected by iothread lock / AioContext.  */
>      BdrvDirtyBitmap *dirty_bitmap;
>      Error *blocker;
>  } BlkMigDevState;
> @@ -100,7 +106,7 @@ typedef struct BlkMigState {
>      int prev_progress;
>      int bulk_completed;
>  
> -    /* Lock must be taken _inside_ the iothread lock.  */
> +    /* Lock must be taken _inside_ the iothread lock and any AioContexts.  */
>      QemuMutex lock;
>  } BlkMigState;
>  
> @@ -264,11 +270,13 @@ static int mig_save_device_bulk(QEMUFile *f, BlkMigDevState *bmds)
>  
>      if (bmds->shared_base) {
>          qemu_mutex_lock_iothread();
> +        aio_context_acquire(bdrv_get_aio_context(bs));
>          while (cur_sector < total_sectors &&
>                 !bdrv_is_allocated(bs, cur_sector, MAX_IS_ALLOCATED_SEARCH,
>                                    &nr_sectors)) {
>              cur_sector += nr_sectors;
>          }
> +        aio_context_release(bdrv_get_aio_context(bs));
>          qemu_mutex_unlock_iothread();
>      }
>  
> @@ -302,11 +310,21 @@ static int mig_save_device_bulk(QEMUFile *f, BlkMigDevState *bmds)
>      block_mig_state.submitted++;
>      blk_mig_unlock();
>  
> +    /* We do not know if bs is under the main thread (and thus does
> +     * not acquire the AioContext when doing AIO) or rather under
> +     * dataplane.  Thus acquire both the iothread mutex and the
> +     * AioContext.
> +     *
> +     * This is ugly and will disappear when we make bdrv_* thread-safe,
> +     * without the need to acquire the AioContext.
> +     */
>      qemu_mutex_lock_iothread();
> +    aio_context_acquire(bdrv_get_aio_context(bmds->bs));
>      blk->aiocb = bdrv_aio_readv(bs, cur_sector, &blk->qiov,
>                                  nr_sectors, blk_mig_read_cb, blk);
>  
>      bdrv_reset_dirty_bitmap(bmds->dirty_bitmap, cur_sector, nr_sectors);
> +    aio_context_release(bdrv_get_aio_context(bmds->bs));
>      qemu_mutex_unlock_iothread();
>  
>      bmds->cur_sector = cur_sector + nr_sectors;
> @@ -321,8 +339,9 @@ static int set_dirty_tracking(void)
>      int ret;
>  
>      QSIMPLEQ_FOREACH(bmds, &block_mig_state.bmds_list, entry) {
> +        /* Creating/dropping dirty bitmaps only requires the big QEMU lock.  */

Why? I don't think it is safe today.  The BDS state is mutated and it can race
with bdrv_set_dirty() etc. (Also the refresh_total_sectors in bdrv_nb_sectors
can even do read/write, no?)

>          bmds->dirty_bitmap = bdrv_create_dirty_bitmap(bmds->bs, BLOCK_SIZE,
>                                                        NULL, NULL);
>          if (!bmds->dirty_bitmap) {
>              ret = -errno;
>              goto fail;
> @@ -332,11 +352,14 @@ static int set_dirty_tracking(void)
>      return ret;
>  }
>  
> +/* Called with iothread lock taken.  */
> +
>  static void unset_dirty_tracking(void)
>  {
>      BlkMigDevState *bmds;
>  
>      QSIMPLEQ_FOREACH(bmds, &block_mig_state.bmds_list, entry) {
> +        /* Creating/dropping dirty bitmaps only requires the big QEMU lock.  */

Ditto.

>          bdrv_release_dirty_bitmap(bmds->bs, bmds->dirty_bitmap);
>      }
>  }
> @@ -597,21 +627,28 @@ static void block_migration_cleanup(void *opaque)
>  {
>      BlkMigDevState *bmds;
>      BlkMigBlock *blk;
> +    AioContext *ctx;
>  
>      bdrv_drain_all();
>  
>      unset_dirty_tracking();
>  
> -    blk_mig_lock();

Why is it okay to skip the blk_mig_lock() for block_mig_state.bmds_list?

>      while ((bmds = QSIMPLEQ_FIRST(&block_mig_state.bmds_list)) != NULL) {
>          QSIMPLEQ_REMOVE_HEAD(&block_mig_state.bmds_list, entry);
>          bdrv_op_unblock_all(bmds->bs, bmds->blocker);
>          error_free(bmds->blocker);
> +
> +        /* Save ctx, because bmds->bs can disappear during bdrv_unref.  */
> +        ctx = bdrv_get_aio_context(bmds->bs);
> +        aio_context_acquire(ctx);
>          bdrv_unref(bmds->bs);
> +        aio_context_release(ctx);
> +
>          g_free(bmds->aio_bitmap);
>          g_free(bmds);
>      }
>  
> +    blk_mig_lock();
>      while ((blk = QSIMPLEQ_FIRST(&block_mig_state.blk_list)) != NULL) {
>          QSIMPLEQ_REMOVE_HEAD(&block_mig_state.blk_list, entry);
>          g_free(blk->buf);