Re: [RFC PATCH-for-5.1] hw/ide: Do not block for AIO while resetting a drive

[John Snow <jsnow@redhat.com>]

On 7/20/20 6:02 AM, Stefan Hajnoczi wrote:
> On Fri, Jul 17, 2020 at 07:19:38PM +0200, Philippe Mathieu-Daudé wrote:
>> Last minute chat:
>> 19:01 <stefanha> f4bug: use bdrv_aio_cancel_async() if possible because it won't block the current thread.
>> 19:02 <stefanha> f4bug: For example, in device emulation code where the guest has requested to cancel an I/O request it's often possible to use the async version.
>> 19:02 <stefanha> f4bug: But in synchronous code like device reset it may be necessary to use the synchronous (blocking) bdrv_aio_cancel() API instead. :(
>> 19:14 <stefanha> f4bug: The way to decide is: will the current function return to the event loop and is there someone who will handle the request completion callback when cancel finishes?
>> 19:14 <stefanha> f4bug: If the next line of code requires the request to finished then async cancel cannot be used.
>> 19:15 <stefanha> f4bug: On the other hand, if the function can return and it's okay for the request to cancel at some future time then you can use async.
>>
>> So I'll revisit this patch :)
>> ---
>>   hw/ide/core.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/hw/ide/core.c b/hw/ide/core.c
>> index d997a78e47..e3a9ce7d25 100644
>> --- a/hw/ide/core.c
>> +++ b/hw/ide/core.c
>> @@ -1315,7 +1315,8 @@ static void ide_reset(IDEState *s)
>>       trace_ide_reset(s);
>>   
>>       if (s->pio_aiocb) {
>> -        blk_aio_cancel(s->pio_aiocb);
>> +        /* If there is a pending AIO callback, invoke it now. */
>> +        blk_aio_cancel_async(s->pio_aiocb);
> 
> This is a place where an async call is not allowed. The completion
> function must be called right away (synchronously) before we can
> continue resetting the device.
> 
> I sent a patch that allows bdrv_aio_cancel() to find the AioContext so
> it can call aio_poll().
> 
> Stefan
> 

OK, dropping Phil's patch here.

--js