Re: [PATCH v7 2/3] io_uring/msg_ring: drop unnecessary submitter_task checks

[Joanne Koong <joannelkoong@gmail.com>]

On Wed, Jan 7, 2026 at 11:07 PM Caleb Sander Mateos
<csander@purestorage.com> wrote:
>
> On Wed, Jan 7, 2026 at 8:25 PM Joanne Koong <joannelkoong@gmail.com> wrote:
> >
> > On Mon, Jan 5, 2026 at 1:05 PM Caleb Sander Mateos
> > <csander@purestorage.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@purestorage.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?
>

Nice, yes, that addresses my concern. I had skipped your changes in patch 1.

Reviewed-by: Joanne Koong <joannelkoong@gmail.com>

Thanks,
Joanne

> Thanks,
> Caleb