Re: [PATCH v4 00/25] block layer: split block APIs in global state and I/O

[Hanna Reitz]

On 18.11.21 14:50, Paolo Bonzini wrote:
> On 11/15/21 17:03, Hanna Reitz wrote:
> > I only really see four solutions for this:
> > (1) We somehow make the amend job run in the main context under the 
> > BQL and have it prevent all concurrent I/O access (seems bad)
> > (2) We can make the permission functions part of the I/O path (seems 
> > wrong and probably impossible?)
> > (3) We can drop the permissions update and permanently require the 
> > permissions that we need when updating keys (I think this might break 
> > existing use cases)
> > (4) We can acquire the BQL around the permission update call and 
> > perhaps that works?
> >
> > I don’t know how (4) would work but it’s basically the only 
> > reasonable solution I can come up with.  Would this be a way to call 
> > a BQL function from an I/O function?
>
> I think that would deadlock:
>
>     main                I/O thread
>     --------            -----
>     start bdrv_co_amend
>                     take BQL
>     bdrv_drain
>     ... hangs ...

:/

Is there really nothing we can do?  Forgive me if I’m talking complete 
nonsense here (because frankly I don’t even really know what a bottom 
half is exactly), but can’t we schedule some coroutine in the main 
thread to do the perm notifications and wait for them in the I/O thread?

> (2) is definitely wrong.
>
> (3) I have no idea.
>
> Would it be possible or meaningful to do the bdrv_child_refresh_perms 
> in qmp_x_blockdev_amend?  It seems that all users need it, and in 
> general it seems weird to amend a qcow2 or luks header (and thus the 
> meaning of parts of the file) while others can write to the same file.

Hmm...  Perhaps.  We would need to undo the permission change when the 
job finishes, though, i.e. in JobDriver.prepare() or JobDriver.clean().  
Doing the change in qmp_x_blockdev_amend() would be asymmetric then, so 
we’d probably want a new JobDriver method that runs in the main thread 
before .run() is invoked. (Unfortunately, “.prepare()” is now taken 
already...)

Doesn’t solve the FUSE problem, but there we could try to just take the 
RESIZE permission permanently and if that fails, we just don’t allow 
truncates for that export.  Not nice, but should work for common cases.

Hanna