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Message-ID: <CADUfDZqYRgMpRATciSzW+Gha_W-RJiX0RYF0K-RLoT_s3OX5qg@mail.gmail.com>
Date: Wed, 7 Jan 2026 23:06:50 -0800
From: Caleb Sander Mateos <csander@...estorage.com>
To: Joanne Koong <joannelkoong@...il.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 Wed, Jan 7, 2026 at 8:25 PM Joanne Koong <joannelkoong@...il.com> wrote:
>
> 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?
Shouldn't patch 1's switch to use smp_store_release() for the clearing
of IORING_SETUP_R_DISABLED and smp_load_acquire() for the check of
IORING_SETUP_R_DISABLED in io_msg_send_fd() ensure the necessary
ordering? Or am I missing something?
Thanks,
Caleb
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