Re: [PATCH v7 13/13] block: Convert 'block_resize' to coroutine

[Kevin Wolf]

Am 15.09.2020 um 16:57 hat Stefan Hajnoczi geschrieben:
> On Wed, Sep 09, 2020 at 05:11:49PM +0200, Kevin Wolf wrote:
> > @@ -2456,8 +2456,7 @@ void qmp_block_resize(bool has_device, const char 
> > *device,
> >          return;
> >      }
> >  
> > -    aio_context = bdrv_get_aio_context(bs);
> > -    aio_context_acquire(aio_context);
> > +    old_ctx = bdrv_co_move_to_aio_context(bs);
> >  
> >      if (size < 0) {
> >          error_setg(errp, QERR_INVALID_PARAMETER_VALUE, "size", "a >0 
> > size");
> 
> Is it safe to call blk_new() outside the BQL since it mutates global state?
> 
> In other words, could another thread race with us?

Hm, probably not.

Would it be safer to have the bdrv_co_move_to_aio_context() call only
immediately before the drain?

> > @@ -2479,8 +2478,8 @@ void qmp_block_resize(bool has_device, const char 
> > *device,
> >      bdrv_drained_end(bs);
> >  
> >  out:
> > +    aio_co_reschedule_self(old_ctx);
> >      blk_unref(blk);
> > -    aio_context_release(aio_context);
> 
> The following precondition is violated by the blk_unref -> bdrv_drain ->
> AIO_WAIT_WHILE() call if blk->refcnt is 1 here:
> 
>  * The caller's thread must be the IOThread that owns @ctx or the main loop
>  * thread (with @ctx acquired exactly once).
> 
> blk_unref() is called from the main loop thread without having acquired
> blk's AioContext.
> 
> Normally blk->refcnt will be > 1 so bdrv_drain() won't be called, but
> I'm not sure if that can be guaranteed.
> 
> The following seems safer although it's uglier:
> 
>   aio_context = bdrv_get_aio_context(bs);
>   aio_context_acquire(aio_context);
>   blk_unref(blk);
>   aio_context_release(aio_context);

May we actually acquire aio_context if blk is in the main thread? I
think we must only do this if it's in a different iothread because we'd
end up with a recursive lock and drain would hang.

Kevin

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