Re: [PATCH] aio: fix a use after free (and fix freeze protection of aio writes)

[Christoph Hellwig]

On Mon, Oct 17, 2016 at 02:19:47PM -0400, Jeff Moyer wrote:
> This ends up being a call to __sb_end_write:
> 
> void __sb_end_write(struct super_block *sb, int level)
> {
>         percpu_up_read(sb->s_writers.rw_sem + level-1);
> }
> 
> Nothing guarantees that submission and completion happen on the same
> CPU.  Is this safe?

Good point.  From my reading of the percpu_rwsem implementation it
is not safe to release it from a different CPU.  Which makes me
wonder how we can protect aio writes properly here..