Re: [Qemu-devel] [PATCH 1/3] block: add BlockBackend->in_flight counter

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Feb 09, 2018 at 06:26:41PM +0100, Paolo Bonzini wrote:
> On 09/02/2018 18:23, Kevin Wolf wrote:
> > Am 09.02.2018 um 17:28 hat Paolo Bonzini geschrieben:
> >> On 08/02/2018 18:18, Stefan Hajnoczi wrote:
> >>> +    BlockDriverState *bs = blk_bs(blk);
> >>> +
> >>> +    if (bs) {
> >>> +        bdrv_drained_begin(bs);
> >>> +    }
> >>> +
> >>> +    /* We may have aio requests like -ENOMEDIUM in flight */
> >>> +    while (atomic_mb_read(&blk->in_flight) > 0) {
> >>> +        aio_poll(blk_get_aio_context(blk), true);
> >>> +    }
> >>> +
> >>> +    if (bs) {
> >>> +        bdrv_drained_end(bs);
> >>
> >> This only works if you are in the default AioContext, otherwise you can
> >> run handlers for one AioContexts in two threads.
> > 
> > Should aio_poll() assert that it's called in the right thread to make
> > sure we obey the rules?
> > 
> >> bdrv_dec_in_flight uses bdrv_wakeup and BDRV_POLL_WHILE to ensure that
> >> this doesn't happen, so there would be more code that you have to copy
> >> into block-backend.c.
> > 
> > Instead of copying, can't we generalise it into a POLL_WHILE(ctx,
> > wakeup, cond) and make BDRV_POLL_WHILE() a wrapper for that?
> 
> Yes, or even move bdrv_wakeup to AioContext would do.  We already have
> block layer-specific fields such as the linux-aio state(*).
> 
> 	(*) though now that linux-aio has been improved to do something
> 	    like epoll, there may be a better reason to place linux-aio
> 	    state in AioContext.

Thanks for the ideas, will fix in v2.

Stefan

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