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

Usama Arif
2022-02-03T23:34:47Z

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);
}

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;
+
+ mutex_lock(&ctx->ev_fd_lock);
+ ev_fd = rcu_dereference_protected(ctx->io_ev_fd, lockdep_is_held(&ctx->ev_fd_lock));
+ if (ev_fd) {
+ ev_fd->unregistering = true;
+ call_rcu(&ev_fd->rcu, io_eventfd_put);
+ mutex_unlock(&ctx->ev_fd_lock);
return 0;
}

+ mutex_unlock(&ctx->ev_fd_lock);
return -ENXIO;
}

@@ -10960,6 +11028,8 @@ 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:
--
2.25.1