Re: [Qemu-devel] [PATCH v3 18/19] block: Use a single global AioWait

[Max Reitz]

On 20.09.18 18:19, Kevin Wolf wrote:
> When draining a block node, we recurse to its parent and for subtree
> drains also to its children. A single AIO_WAIT_WHILE() is then used to
> wait for bdrv_drain_poll() to become true, which depends on all of the
> nodes we recursed to. However, if the respective child or parent becomes
> quiescent and calls bdrv_wakeup(), only the AioWait of the child/parent
> is checked, while AIO_WAIT_WHILE() depends on the AioWait of the
> original node.
> 
> Fix this by using a single AioWait for all callers of AIO_WAIT_WHILE().
> 
> This may mean that the draining thread gets a few more unnecessary
> wakeups because an unrelated operation got completed, but we already
> wake it up when something _could_ have changed rather than only if it
> has certainly changed.
> 
> Apart from that, drain is a slow path anyway. In theory it would be
> possible to use wakeups more selectively and still correctly, but the
> gains are likely not worth the additional complexity. In fact, this
> patch is a nice simplification for some places in the code.
> 
> Signed-off-by: Kevin Wolf <address@hidden>
> ---
>  include/block/aio-wait.h  | 11 +++++------
>  include/block/block.h     |  6 +-----
>  include/block/block_int.h |  3 ---
>  include/block/blockjob.h  | 10 ----------
>  block.c                   |  5 -----
>  block/block-backend.c     | 11 ++++-------
>  block/io.c                |  7 ++-----
>  blockjob.c                | 13 +------------
>  job.c                     |  3 +--
>  util/aio-wait.c           | 11 ++++++-----
>  10 files changed, 20 insertions(+), 60 deletions(-)

Reviewed-by: Max Reitz <address@hidden>

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