Re: [PATCH v5 05/31] block-backend: special comments for blk_set/get_perm due to fuse

[Emanuele Giuseppe Esposito]

On 10/12/2021 15:38, Hanna Reitz wrote:
> On 24.11.21 07:43, Emanuele Giuseppe Esposito wrote:
> > Fuse logic can be classified as I/O, so there is no BQL held
> > during its execution. And yet, it uses blk_{get/set}_perm
> > functions, that are classified as BQL and clearly require
> > the BQL lock. Since there is no easy solution for this,
> > add a couple of TODOs and FIXME in the relevant sections of the
> > code.
>
> Well, the problem goes deeper, because we still consider them GS 
> functions.  So it’s fine for the permission function 
> raw_handle_perm_lock() to call bdrv_get_flags(), which is a GS function, 
> and then you still get:
>
> qemu-storage-daemon: ../block.c:6195: bdrv_get_flags: Assertion 
> `qemu_in_main_thread()' failed.
>
> It looks like in this case making bdrv_get_flags() not a GS function 
> would be feasible and might solve the problem, but I guess we should 
> instead think about adding something like
>
> if (!exp->growable && !qemu_in_main_thread()) {
>      /* Changing permissions like below only works in the main thread */
>      return -EPERM;
> }
>
> to fuse_do_truncate().
>
> Ideally, to make up for this, we should probably take the RESIZE 
> permission in fuse_export_create() for writable exports in iothreads.

I think I got it. I will send the RESIZE permission fix in a separate patch.

Thank you,
Emanuele