On Fri, Mar 06, 2020 at 03:17:40PM +0100, Paolo Bonzini wrote:
> On 06/03/20 14:50, Stefan Hajnoczi wrote:
> >> Not sure I understand the "almost" part.  If it's accessed only from
> >> aio_poll() it is protected via either AIO_WAIT_WHILE or the BQL, not by
> >> ctx->list_lock; if it's protected by ctx->list_lock (using
> >> qemu_lockcnt_inc in readers), it is an RCU list.
> > aio_remove_fd_handler() removes nodes from the list during
> > aio_set_fd_handler(), but only while holding ctx->list_lock and the
> > count is zero (no readers).
> > 
> > All other access is done from with ctx->list_lock incremented.  This
> > code needs to be reentrant in case of nested aio_poll() but nothing else
> > will access the list at the same time.
> 
> Oh, I see, adds are only done under ctx->list_lock and those are the
> part that need the write barriers in the RCU iterators.

I'll update the comment when merging this series.

Stefan