Re: [PATCH 7/7] aio-posix: remove idle poll handlers to improve scalability

[Paolo Bonzini]

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.

Paolo

> Therefore RCU is not needed.  ctx->list_lock acts more like a rwlock.