Commit 77bc59b4 authored by Usama Arif's avatar Usama Arif Committed by Jens Axboe
Browse files

io_uring: avoid ring quiesce while registering/unregistering eventfd 



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 already done under
uring_lock so multiple threads won'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_register
with IORING_REGISTER_EVENTFD takes less than 1ms with RCU lock, compared
to 15ms before with ring quiesce.

Signed-off-by: default avatarUsama Arif <usama.arif@bytedance.com>
Link: https://lore.kernel.org/r/20220204145117.1186568-3-usama.arif@bytedance.com


[axboe: long line fixups]
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 2757be22

fs/io_uring.c

+68 −21
Original line number Diff line number Diff line
@@ -326,6 +326,11 @@ struct io_submit_state { 
	struct blk_plug		plug;

};



struct io_ev_fd {

	struct eventfd_ctx	*cq_ev_fd;

	struct rcu_head		rcu;

};



struct io_ring_ctx {

	/* const or read-mostly hot data */

	struct {
@@ -399,7 +404,7 @@ 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 wait_queue_head	cq_wait;

		unsigned		cq_extra;

		atomic_t		cq_timeouts;
@@ -1726,13 +1731,36 @@ 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 */
@@ -9361,31 +9387,50 @@ 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)

	ev_fd = rcu_dereference_protected(ctx->io_ev_fd,

					lockdep_is_held(&ctx->uring_lock));

	if (ev_fd)

		return -EBUSY;



	if (copy_from_user(&fd, fds, sizeof(*fds)))

		return -EFAULT;



	ctx->cq_ev_fd = eventfd_ctx_fdget(fd);

	if (IS_ERR(ctx->cq_ev_fd)) {

		int ret = PTR_ERR(ctx->cq_ev_fd);

	ev_fd = kmalloc(sizeof(*ev_fd), GFP_KERNEL);

	if (!ev_fd)

		return -ENOMEM;



		ctx->cq_ev_fd = NULL;

	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);

		return ret;

	}



	return 0;

	rcu_assign_pointer(ctx->io_ev_fd, ev_fd);

	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);



	eventfd_ctx_put(ev_fd->cq_ev_fd);

	kfree(ev_fd);

}



static int io_eventfd_unregister(struct io_ring_ctx *ctx)

{

	if (ctx->cq_ev_fd) {

		eventfd_ctx_put(ctx->cq_ev_fd);

		ctx->cq_ev_fd = NULL;

	struct io_ev_fd *ev_fd;



	ev_fd = rcu_dereference_protected(ctx->io_ev_fd,

					lockdep_is_held(&ctx->uring_lock));

	if (ev_fd) {

		rcu_assign_pointer(ctx->io_ev_fd, NULL);

		call_rcu(&ev_fd->rcu, io_eventfd_put);

		return 0;

	}
@@ -9450,8 +9495,8 @@ static __cold void io_ring_ctx_free(struct io_ring_ctx *ctx) 
		__io_sqe_files_unregister(ctx);

	if (ctx->rings)

		__io_cqring_overflow_flush(ctx, true);

	mutex_unlock(&ctx->uring_lock);

	io_eventfd_unregister(ctx);

	mutex_unlock(&ctx->uring_lock);

	io_destroy_buffers(ctx);

	if (ctx->sq_creds)

		put_cred(ctx->sq_creds);
@@ -10968,6 +11013,8 @@ static bool io_register_op_must_quiesce(int op) 
	case IORING_REGISTER_FILES:

	case IORING_UNREGISTER_FILES:

	case IORING_REGISTER_FILES_UPDATE:

	case IORING_REGISTER_EVENTFD:

	case IORING_UNREGISTER_EVENTFD:

	case IORING_REGISTER_PROBE:

	case IORING_REGISTER_PERSONALITY:

	case IORING_UNREGISTER_PERSONALITY: