[PATCH v6 3/5] io_uring: avoid ring quiesce while registering async eventfd

From: Usama Arif
Date: Fri Feb 04 2022 - 09:51:34 EST


This is done using the RCU data structure (io_ev_fd). eventfd_async
is moved from io_ring_ctx to io_ev_fd which is RCU protected hence
avoiding ring quiesce which is much more expensive than an RCU lock.
The place where eventfd_async is read is already under rcu_read_lock
so there is no extra RCU read-side critical section needed.

Signed-off-by: Usama Arif <usama.arif@xxxxxxxxxxxxx>
---
fs/io_uring.c | 22 ++++++++++++----------
1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index ad6361aeaca7..671c57f9c1fa 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -328,6 +328,7 @@ struct io_submit_state {

struct io_ev_fd {
struct eventfd_ctx *cq_ev_fd;
+ unsigned int eventfd_async: 1;
struct rcu_head rcu;
};

@@ -340,7 +341,6 @@ struct io_ring_ctx {
unsigned int flags;
unsigned int compat: 1;
unsigned int drain_next: 1;
- unsigned int eventfd_async: 1;
unsigned int restricted: 1;
unsigned int off_timeout_used: 1;
unsigned int drain_active: 1;
@@ -1752,7 +1752,7 @@ static void io_eventfd_signal(struct io_ring_ctx *ctx)
if (READ_ONCE(ctx->rings->cq_flags) & IORING_CQ_EVENTFD_DISABLED)
goto out;

- if (!ctx->eventfd_async || io_wq_current_is_worker())
+ if (!ev_fd->eventfd_async || io_wq_current_is_worker())
eventfd_signal(ev_fd->cq_ev_fd, 1);

out:
@@ -9373,7 +9373,8 @@ static int __io_sqe_buffers_update(struct io_ring_ctx *ctx,
return done ? done : err;
}

-static int io_eventfd_register(struct io_ring_ctx *ctx, void __user *arg)
+static int io_eventfd_register(struct io_ring_ctx *ctx, void __user *arg,
+ unsigned int eventfd_async)
{
struct io_ev_fd *ev_fd;
__s32 __user *fds = arg;
@@ -9396,6 +9397,7 @@ static int io_eventfd_register(struct io_ring_ctx *ctx, void __user *arg)
kfree(ev_fd);
return ret;
}
+ ev_fd->eventfd_async = eventfd_async;

rcu_assign_pointer(ctx->io_ev_fd, ev_fd);
return ret;
@@ -11000,6 +11002,7 @@ static bool io_register_op_must_quiesce(int op)
case IORING_UNREGISTER_FILES:
case IORING_REGISTER_FILES_UPDATE:
case IORING_REGISTER_EVENTFD:
+ case IORING_REGISTER_EVENTFD_ASYNC:
case IORING_UNREGISTER_EVENTFD:
case IORING_REGISTER_PROBE:
case IORING_REGISTER_PERSONALITY:
@@ -11100,17 +11103,16 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
ret = io_register_files_update(ctx, arg, nr_args);
break;
case IORING_REGISTER_EVENTFD:
- case IORING_REGISTER_EVENTFD_ASYNC:
ret = -EINVAL;
if (nr_args != 1)
break;
- ret = io_eventfd_register(ctx, arg);
- if (ret)
+ ret = io_eventfd_register(ctx, arg, 0);
+ break;
+ case IORING_REGISTER_EVENTFD_ASYNC:
+ ret = -EINVAL;
+ if (nr_args != 1)
break;
- if (opcode == IORING_REGISTER_EVENTFD_ASYNC)
- ctx->eventfd_async = 1;
- else
- ctx->eventfd_async = 0;
+ ret = io_eventfd_register(ctx, arg, 1);
break;
case IORING_UNREGISTER_EVENTFD:
ret = -EINVAL;
--
2.25.1