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Message-ID: <CAJnrk1YBQdCgOHG2T_D6wV_94kLLftP_G6sLyocRf2wCLTsweg@mail.gmail.com>
Date: Wed, 7 Jan 2026 20:25:32 -0800
From: Joanne Koong <joannelkoong@...il.com>
To: Caleb Sander Mateos <csander@...estorage.com>
Cc: Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 2/3] io_uring/msg_ring: drop unnecessary submitter_task checks
On Mon, Jan 5, 2026 at 1:05 PM Caleb Sander Mateos
<csander@...estorage.com> wrote:
>
> __io_msg_ring_data() checks that the target_ctx isn't
> IORING_SETUP_R_DISABLED before calling io_msg_data_remote(), which calls
> io_msg_remote_post(). So submitter_task can't be modified concurrently
> with the read in io_msg_remote_post(). Additionally, submitter_task must
> exist, as io_msg_data_remote() is only called for io_msg_need_remote(),
> i.e. task_complete is set, which requires IORING_SETUP_DEFER_TASKRUN,
> which in turn requires IORING_SETUP_SINGLE_ISSUER. And submitter_task is
> assigned in io_uring_create() or io_register_enable_rings() before
> enabling any IORING_SETUP_SINGLE_ISSUER io_ring_ctx.
> Similarly, io_msg_send_fd() checks IORING_SETUP_R_DISABLED and
> io_msg_need_remote() before calling io_msg_fd_remote(). submitter_task
> therefore can't be modified concurrently with the read in
> io_msg_fd_remote() and must be non-null.
> io_register_enable_rings() can't run concurrently because it's called
> from io_uring_register() -> __io_uring_register() with uring_lock held.
> Thus, replace the READ_ONCE() and WRITE_ONCE() of submitter_task with
> plain loads and stores. And remove the NULL checks of submitter_task in
> io_msg_remote_post() and io_msg_fd_remote().
>
> Signed-off-by: Caleb Sander Mateos <csander@...estorage.com>
> ---
> io_uring/io_uring.c | 7 +------
> io_uring/msg_ring.c | 18 +++++-------------
> io_uring/register.c | 2 +-
> 3 files changed, 7 insertions(+), 20 deletions(-)
>
> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
> index ec27fafcb213..b31d88295297 100644
> --- a/io_uring/io_uring.c
> +++ b/io_uring/io_uring.c
> @@ -3663,17 +3663,12 @@ static __cold int io_uring_create(struct io_ctx_config *config)
> ret = -EFAULT;
> goto err;
> }
>
> if (ctx->flags & IORING_SETUP_SINGLE_ISSUER
> - && !(ctx->flags & IORING_SETUP_R_DISABLED)) {
> - /*
> - * Unlike io_register_enable_rings(), don't need WRITE_ONCE()
> - * since ctx isn't yet accessible from other tasks
> - */
> + && !(ctx->flags & IORING_SETUP_R_DISABLED))
> ctx->submitter_task = get_task_struct(current);
> - }
>
> file = io_uring_get_file(ctx);
> if (IS_ERR(file)) {
> ret = PTR_ERR(file);
> goto err;
> diff --git a/io_uring/msg_ring.c b/io_uring/msg_ring.c
> index 87b4d306cf1b..57ad0085869a 100644
> --- a/io_uring/msg_ring.c
> +++ b/io_uring/msg_ring.c
> @@ -78,26 +78,21 @@ static void io_msg_tw_complete(struct io_tw_req tw_req, io_tw_token_t tw)
> io_add_aux_cqe(ctx, req->cqe.user_data, req->cqe.res, req->cqe.flags);
> kfree_rcu(req, rcu_head);
> percpu_ref_put(&ctx->refs);
> }
>
> -static int io_msg_remote_post(struct io_ring_ctx *ctx, struct io_kiocb *req,
> +static void io_msg_remote_post(struct io_ring_ctx *ctx, struct io_kiocb *req,
> int res, u32 cflags, u64 user_data)
> {
> - if (!READ_ONCE(ctx->submitter_task)) {
> - kfree_rcu(req, rcu_head);
> - return -EOWNERDEAD;
> - }
> req->opcode = IORING_OP_NOP;
> req->cqe.user_data = user_data;
> io_req_set_res(req, res, cflags);
> percpu_ref_get(&ctx->refs);
> req->ctx = ctx;
> req->tctx = NULL;
> req->io_task_work.func = io_msg_tw_complete;
> io_req_task_work_add_remote(req, IOU_F_TWQ_LAZY_WAKE);
> - return 0;
> }
>
> static int io_msg_data_remote(struct io_ring_ctx *target_ctx,
> struct io_msg *msg)
> {
> @@ -109,12 +104,12 @@ static int io_msg_data_remote(struct io_ring_ctx *target_ctx,
> return -ENOMEM;
>
> if (msg->flags & IORING_MSG_RING_FLAGS_PASS)
> flags = msg->cqe_flags;
>
> - return io_msg_remote_post(target_ctx, target, msg->len, flags,
> - msg->user_data);
> + io_msg_remote_post(target_ctx, target, msg->len, flags, msg->user_data);
> + return 0;
> }
>
> static int __io_msg_ring_data(struct io_ring_ctx *target_ctx,
> struct io_msg *msg, unsigned int issue_flags)
> {
> @@ -125,11 +120,11 @@ static int __io_msg_ring_data(struct io_ring_ctx *target_ctx,
> return -EINVAL;
> if (!(msg->flags & IORING_MSG_RING_FLAGS_PASS) && msg->dst_fd)
> return -EINVAL;
> /*
> * Keep IORING_SETUP_R_DISABLED check before submitter_task load
> - * in io_msg_data_remote() -> io_msg_remote_post()
> + * in io_msg_data_remote() -> io_req_task_work_add_remote()
> */
> if (smp_load_acquire(&target_ctx->flags) & IORING_SETUP_R_DISABLED)
> return -EBADFD;
>
> if (io_msg_need_remote(target_ctx))
> @@ -225,14 +220,11 @@ static void io_msg_tw_fd_complete(struct callback_head *head)
>
> static int io_msg_fd_remote(struct io_kiocb *req)
> {
> struct io_ring_ctx *ctx = req->file->private_data;
> struct io_msg *msg = io_kiocb_to_cmd(req, struct io_msg);
> - struct task_struct *task = READ_ONCE(ctx->submitter_task);
> -
> - if (unlikely(!task))
> - return -EOWNERDEAD;
> + struct task_struct *task = ctx->submitter_task;
Is the if !task check here still needed? in the
io_register_enable_rings() logic I see
if (ctx->flags & IORING_SETUP_SINGLE_ISSUER && !ctx->submitter_task) {
ctx->submitter_task = get_task_struct(current);
...
}
and then a few lines below
ctx->flags &= ~IORING_SETUP_R_DISABLED;
but I'm not seeing any memory barrier stuff that prevents these from
being reordered.
In io_msg_send_fd() I see that we check "if (target_ctx->flags &
IORING_SETUP_R_DISABLED) return -EBADFD;" before calling into
io_msg_fd_remote() here but if the ctx->submitter_task assignment and
IORING_SETUP_R_DISABLED flag clearing logic are reordered, then it
seems like this opens a race condition where there could be a null ptr
crash when task_work_add() gets called below?
Thanks,
Joanne
>
> init_task_work(&msg->tw, io_msg_tw_fd_complete);
> if (task_work_add(task, &msg->tw, TWA_SIGNAL))
> return -EOWNERDEAD;
>
> diff --git a/io_uring/register.c b/io_uring/register.c
> index 12318c276068..8104728af294 100644
> --- a/io_uring/register.c
> +++ b/io_uring/register.c
> @@ -179,11 +179,11 @@ static int io_register_enable_rings(struct io_ring_ctx *ctx)
> {
> if (!(ctx->flags & IORING_SETUP_R_DISABLED))
> return -EBADFD;
>
> if (ctx->flags & IORING_SETUP_SINGLE_ISSUER && !ctx->submitter_task) {
> - WRITE_ONCE(ctx->submitter_task, get_task_struct(current));
> + ctx->submitter_task = get_task_struct(current);
> /*
> * Lazy activation attempts would fail if it was polled before
> * submitter_task is set.
> */
> if (wq_has_sleeper(&ctx->poll_wq))
> --
> 2.45.2
>
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