Re: [PATCH v3 07/13] epoll: call ep_add_event_to_uring() from ep_poll_callback()

Peter Zijlstra
2019-05-31T09:56:23Z

On Thu, May 16, 2019 at 10:58:04AM +0200, Roman Penyaev wrote:
> Each ep_poll_callback() is called when fd calls wakeup() on epfd.
> So account new event in user ring.
>
> The tricky part here is EPOLLONESHOT. Since we are lockless we
> have to be deal with ep_poll_callbacks() called in paralle, thus
> use cmpxchg to clear public event bits and filter out concurrent
> call from another cpu.
>
> Signed-off-by: Roman Penyaev <rpenyaev@suse.de>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: linux-fsdevel@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
>
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index 2f551c005640..55612da9651e 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -1407,6 +1407,29 @@ struct file *get_epoll_tfile_raw_ptr(struct file *file, int tfd,
> }
> #endif /* CONFIG_CHECKPOINT_RESTORE */
>
> +/**
> + * Atomically clear public event bits and return %true if the old value has
> + * public event bits set.
> + */
> +static inline bool ep_clear_public_event_bits(struct epitem *epi)
> +{
> + __poll_t old, flags;
> +
> + /*
> + * Here we race with ourselves and with ep_modify(), which can
> + * change the event bits. In order not to override events updated
> + * by ep_modify() we have to do cmpxchg.
> + */
> +
> + old = epi->event.events;
> + do {
> + flags = old;
> + } while ((old = cmpxchg(&epi->event.events, flags,
> + flags & EP_PRIVATE_BITS)) != flags);
> +
> + return flags & ~EP_PRIVATE_BITS;
> +}

AFAICT epi->event.events also has normal writes to it, eg. in
ep_modify(). A number of architectures cannot handle concurrent normal
writes and cmpxchg() to the same variable.