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

[Emanuele Giuseppe Esposito <eesposit@redhat.com>]

On 19/11/2021 04:13, Paolo Bonzini wrote:
> 
> 
> El jue., 18 nov. 2021 16:31, Hanna Reitz <hreitz@redhat.com 
> <mailto:hreitz@redhat.com>> escribió:
> 
>     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?
> 
> 
> I think you still get a deadlock, just one with a longer chain. You 
> still have a cycle of things depending on each other, but one of them is 
> now the I/O thread waiting for the bottom half.
> 
>     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...)
> 
> 
> Ok at least it's feasible.

Ok I think I got it. I will create a new callback, maybe "pre_run" or 
something like that to perform the first bdrv_child_refresh_perms and 
implement the .clean callback to perform the "cleanup" 
bdrv_child_refresh_perms in block_crypto_amend_options_generic_luks.

> 
>     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.
> 
> 
> Yeah definitely not nice. Probably permissions could be protected by 
> their own mutex, even a global one like the one we have for jobs. For 
> now I suggest just ignoring the problem and adding a comment, since it's 
> not really something that didn't exist.
> 

Will add a TODO in blk_set/get permissions explaining the issue.
Last issue we had with regards to permissions in GS had to do with 
bdrv_co_invalidate_cache: however, Paolo suggested me a simple fix to 
simply assert that the function is either under BQL or does not have 
open_flags & BDRV_O_INACTIVE set. This basically skips the permission 
code block, entering it only if we have the BQL.


Ok, apart from this permissions issue and assert_bdrv_graph_writable, I 
should have addressed all main comments of this series. Assume that for 
the others where I did not explicitly answered, I agree and applied your 
comments.

Thank you,
Emanuele