Re: [Qemu-devel] [PATCH v3 0/8] block: Asynchronous request cancellation

[Stefan Hajnoczi]

On Wed, Aug 27, 2014 at 10:49:08AM +0800, Fam Zheng wrote:
> v3: Drop "RFC".
>     Improvements according to Paolo's comments:
>     05: Just use THREAD_DONE and ret = -ECANCELED in thread-pool.c
>     06: Don't check dbs->cancelled for twice.
>         Don't set dbs->acb to NULL.
> 
> v2: Drop the unfinished scsi part, which was broken in v1. (Paolo)
>     Add refcnt in BlockDriverAIOCB to maintain invariant of acb availability 
> in
>     bdrv_aio_cancel_async. (Paolo)
>     Drop blkdebug change. (Stefan)
> 
> This series adds a new block layer API:
> 
>   void bdrv_aio_cancel_async(BlockDriverAIOCB *acb);
> 
> which is similar to existing bdrv_aio_cancel in that it cancels an AIO 
> request,
> but different that it doesn't block until the request is completely cancelled
> or done.
> 
> More importantly, the completion callback, BlockDriverAIOCB.cb, is guaranteed
> to be called, so that the cb can take care of resource releasing and status
> reporting to guest, etc.
> 
> In the following work, scsi emulation code will be shifted to use the async
> cancelling.
> 
> One major benefit would be that when guest tries to cancel a request, where 
> the
> request cannot be cancelled easily, (due to throttled BlockDriverState, a lost
> connection, or a large request queue), we don't need to block the whole vm 
> with
> a busy loop, which is how bdrv_aio_cancel is implemented now.
> 
> A test case that is easy to reproduce is, throttle a scsi-disk to a very low
> limit, for example 50 bps, then stress the guest block device with dd or fio.
> 
> Currently, the vm will quickly hang when it loses patience and send a tmf
> command to cancel the request, at which point we will busy wait in
> bdrv_aio_cancel, until the request is slowly spit out from throttled_reqs.
> 
> Later, we will change scsi device code to make this asynchronous, on top of
> bdrv_aio_cancel_async.

We need to get rid of .bdrv_aio_cancel().  Keeping both
.bdrv_aio_cancel() and .bdrv_aio_cancel_async() around is problematic
because they have slightly different semantics.

This patch series makes block driver cancellation more complex because
we support both approaches :(.

Could we do something like:

void bdrv_aio_cancel(BdrvAIOCB *acb)
{
    bdrv_aiocb_ref(acb);
    bdrv_aio_cancel_async(acb);
    while (acb->ref > 1) {
        aio_poll(bdrv_get_aio_context(acb->bs), true);
    }
    bdrv_aiocb_release(acb);
}

(pseudo-code)

Then .bdrv_aio_cancel() should be deleted.

Stefan

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