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

Jens Axboe
2022-02-04T00:12:30Z

On 2/3/22 4:46 PM, Pavel Begunkov wrote:
> 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?

Agree on both, for this scheme to work it's also critical that we assign
evfd = NULL before call_rcu() is done, not in the callback. Ditto on the
lock, the register side is under uring_lock anyway, so this one doesn't
add anything.

--
Jens Axboe