Re: [PATCH v5 2/4] io_uring: avoid ring quiesce while registering/unregistering eventfd

Pavel Begunkov
2022-02-03T23:51:10Z

On 2/3/22 23:34, Usama Arif wrote:
> This is done by creating a new RCU data structure (io_ev_fd) as part of
> io_ring_ctx that holds the eventfd_ctx.
>
> The function io_eventfd_signal is executed under rcu_read_lock with a
> single rcu_dereference to io_ev_fd so that if another thread unregisters
> the eventfd while io_eventfd_signal is still being executed, the
> eventfd_signal for which io_eventfd_signal was called completes
> successfully.
>
> The process of registering/unregistering eventfd is done under a lock
> so multiple threads don't enter a race condition while
> registering/unregistering eventfd.
>
> With the above approach ring quiesce can be avoided which is much more
> expensive then using RCU lock. On the system tested, io_uring_reigster with
> IORING_REGISTER_EVENTFD takes less than 1ms with RCU lock, compared to 15ms
> before with ring quiesce.
>
> Signed-off-by: Usama Arif <usama.arif@bytedance.com>
> ---
> fs/io_uring.c | 116 ++++++++++++++++++++++++++++++++++++++++----------
> 1 file changed, 93 insertions(+), 23 deletions(-)
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 21531609a9c6..51602bddb9a8 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -326,6 +326,13 @@ struct io_submit_state {
> struct blk_plug plug;
> };
>
> +struct io_ev_fd {
> + struct eventfd_ctx *cq_ev_fd;
> + struct io_ring_ctx *ctx;
> + struct rcu_head rcu;
> + bool unregistering;
> +};
> +
> struct io_ring_ctx {
> /* const or read-mostly hot data */
> struct {
> @@ -399,7 +406,8 @@ struct io_ring_ctx {
> struct {
> unsigned cached_cq_tail;
> unsigned cq_entries;
> - struct eventfd_ctx *cq_ev_fd;
> + struct io_ev_fd __rcu *io_ev_fd;
> + struct mutex ev_fd_lock;
> struct wait_queue_head cq_wait;
> unsigned cq_extra;
> atomic_t cq_timeouts;
> @@ -1448,6 +1456,7 @@ static __cold struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
> xa_init_flags(&ctx->io_buffers, XA_FLAGS_ALLOC1);
> xa_init_flags(&ctx->personalities, XA_FLAGS_ALLOC1);
> mutex_init(&ctx->uring_lock);
> + mutex_init(&ctx->ev_fd_lock);
> init_waitqueue_head(&ctx->cq_wait);
> spin_lock_init(&ctx->completion_lock);
> spin_lock_init(&ctx->timeout_lock);
> @@ -1726,13 +1735,32 @@ static inline struct io_uring_cqe *io_get_cqe(struct io_ring_ctx *ctx)
> return &rings->cqes[tail & mask];
> }
>
> -static inline bool io_should_trigger_evfd(struct io_ring_ctx *ctx)
> +static void io_eventfd_signal(struct io_ring_ctx *ctx)
> {
> - if (likely(!ctx->cq_ev_fd))
> - return false;
> + struct io_ev_fd *ev_fd;
> +
> + /* Return quickly if ctx->io_ev_fd doesn't exist */
> + if (likely(!rcu_dereference_raw(ctx->io_ev_fd)))
> + return;
> +
> + rcu_read_lock();
> + /* rcu_dereference ctx->io_ev_fd once and use it for both for checking and eventfd_signal */
> + ev_fd = rcu_dereference(ctx->io_ev_fd);
> +
> + /*
> + * Check again if ev_fd exists incase an io_eventfd_unregister call completed between
> + * the NULL check of ctx->io_ev_fd at the start of the function and rcu_read_lock.
> + */
> + if (unlikely(!ev_fd))
> + goto out;
> if (READ_ONCE(ctx->rings->cq_flags) & IORING_CQ_EVENTFD_DISABLED)
> - return false;
> - return !ctx->eventfd_async || io_wq_current_is_worker();
> + goto out;
> +
> + if (!ctx->eventfd_async || io_wq_current_is_worker())
> + eventfd_signal(ev_fd->cq_ev_fd, 1);
> +
> +out:
> + rcu_read_unlock();
> }
>
> /*
> @@ -1751,8 +1779,7 @@ static void io_cqring_ev_posted(struct io_ring_ctx *ctx)
> */
> if (wq_has_sleeper(&ctx->cq_wait))
> wake_up_all(&ctx->cq_wait);
> - if (io_should_trigger_evfd(ctx))
> - eventfd_signal(ctx->cq_ev_fd, 1);
> + io_eventfd_signal(ctx);
> }
>
> static void io_cqring_ev_posted_iopoll(struct io_ring_ctx *ctx)
> @@ -1764,8 +1791,7 @@ static void io_cqring_ev_posted_iopoll(struct io_ring_ctx *ctx)
> if (waitqueue_active(&ctx->cq_wait))
> wake_up_all(&ctx->cq_wait);
> }
> - if (io_should_trigger_evfd(ctx))
> - eventfd_signal(ctx->cq_ev_fd, 1);
> + io_eventfd_signal(ctx);
> }
>
> /* Returns true if there are no backlogged entries after the flush */
> @@ -9353,34 +9379,76 @@ static int __io_sqe_buffers_update(struct io_ring_ctx *ctx,
>
> static int io_eventfd_register(struct io_ring_ctx *ctx, void __user *arg)
> {
> + struct io_ev_fd *ev_fd;
> __s32 __user *fds = arg;
> - int fd;
> + int fd, ret;
>
> - if (ctx->cq_ev_fd)
> - return -EBUSY;
> + mutex_lock(&ctx->ev_fd_lock);
> + ret = -EBUSY;
> + ev_fd = rcu_dereference_protected(ctx->io_ev_fd, lockdep_is_held(&ctx->ev_fd_lock));
> + if (ev_fd) {
> + /*
> + * If ev_fd exists, there are 2 possibilities:
> + * - The rcu_callback to io_eventfd_put hasn't finished while unregistering
> + * (hence ev_fd->unregistering is true) and io_eventfd_register
> + * can continue and overwrite ctx->io_ev_fd with the new eventfd.
> + * - Or io_eventfd_register has been called on an io_uring that has
> + * already registered a valid eventfd in which case return -EBUSY.
> + */
> + if(!ev_fd->unregistering)
> + goto out;
> + }
>
> + ret = -EFAULT;
> if (copy_from_user(&fd, fds, sizeof(*fds)))
> - return -EFAULT;
> + goto out;
>
> - ctx->cq_ev_fd = eventfd_ctx_fdget(fd);
> - if (IS_ERR(ctx->cq_ev_fd)) {
> - int ret = PTR_ERR(ctx->cq_ev_fd);
> + ret = -ENOMEM;
> + ev_fd = kmalloc(sizeof(*ev_fd), GFP_KERNEL);
> + if (!ev_fd)
> + goto out;
>
> - ctx->cq_ev_fd = NULL;
> - return ret;
> + ev_fd->cq_ev_fd = eventfd_ctx_fdget(fd);
> + if (IS_ERR(ev_fd->cq_ev_fd)) {
> + ret = PTR_ERR(ev_fd->cq_ev_fd);
> + kfree(ev_fd);
> + goto out;
> }
> + ev_fd->ctx = ctx;
> + ev_fd->unregistering = false;
>
> - return 0;
> + rcu_assign_pointer(ctx->io_ev_fd, ev_fd);
> + ret = 0;
> +
> +out:
> + mutex_unlock(&ctx->ev_fd_lock);
> + return ret;
> +}
> +
> +static void io_eventfd_put(struct rcu_head *rcu)
> +{
> + struct io_ev_fd *ev_fd = container_of(rcu, struct io_ev_fd, rcu);
> + struct io_ring_ctx *ctx = ev_fd->ctx;
> +
> + eventfd_ctx_put(ev_fd->cq_ev_fd);
> + kfree(ev_fd);
> + rcu_assign_pointer(ctx->io_ev_fd, NULL);
> }

Emm, it happens after the grace period, so you have a gap where a
request may read a freed eventfd... What I think you wanted to do
is more like below:

io_eventfd_put() {
evfd = ...;
eventfd_ctx_put(evfd->evfd);
kfree(io_ev_fd);
}

register() {
mutex_lock();
ev_fd = rcu_deref();
if (ev_fd) {
rcu_assign_pointer(ctx->evfd, NULL);
call_rcu(&ev_fd->evfd, io_eventfd_put);
}
mutex_unlock();
}

Note, there's no need in ->unregistering. I also doubt you need
->ev_fd_lock, how about just using already taken ->uring_lock?

--
Pavel Begunkov