Re: [PATCH] RDMA/rxe: Fix race in do_task() when draining

[Gui-Dong Han]

On Thu, Sep 18, 2025 at 5:58 PM Leon Romanovsky <leon@xxxxxxxxxx> wrote:
>
> On Wed, Sep 17, 2025 at 12:30:56PM -0700, yanjun.zhu wrote:
> > On 9/17/25 3:06 AM, Gui-Dong Han wrote:
> > > When do_task() exhausts its RXE_MAX_ITERATIONS budget, it unconditionally
> >
> > From the source code, it will check ret value, then set it to
> > TASK_STATE_IDLE, not unconditionally.
> >
> > > sets the task state to TASK_STATE_IDLE to reschedule. This overwrites
> > > the TASK_STATE_DRAINING state that may have been concurrently set by
> > > rxe_cleanup_task() or rxe_disable_task().
> >
> > From the source code, there is a spin lock to protect the state. It will not
> > make race condition.
> >
> > >
> > > This race condition breaks the cleanup and disable logic, which expects
> > > the task to stop processing new work. The cleanup code may proceed while
> > > do_task() reschedules itself, leading to a potential use-after-free.
> > >
> >
> > Can you post the call trace when this problem occurred?
> >
> > Hi, Jason && Leon
> >
> > Please comment on this problem.
>
> The idea to recheck task->state looks correct to me, otherwise we overwrite it unconditionally.
> However I would write this patch slightly different (without cont = 1):
>
> diff --git a/drivers/infiniband/sw/rxe/rxe_task.c b/drivers/infiniband/sw/rxe/rxe_task.c
> index 6f8f353e95838..2ff5d7cc0a933 100644
> --- a/drivers/infiniband/sw/rxe/rxe_task.c
> +++ b/drivers/infiniband/sw/rxe/rxe_task.c
> @@ -132,8 +132,10 @@ static void do_task(struct rxe_task *task)
>                  * yield the cpu and reschedule the task
>                  */
>                 if (!ret) {
> -                       task->state = TASK_STATE_IDLE;
> -                       resched = 1;
> +                       if (task->state != TASK_STATE_DRAINING) {
> +                               task->state = TASK_STATE_IDLE;
> +                               resched = 1;
> +                       }
>                         goto exit;
>                 }
>
> @@ -151,7 +153,6 @@ static void do_task(struct rxe_task *task)
>                         break;
>
>                 case TASK_STATE_DRAINING:
> -                       task->state = TASK_STATE_DRAINED;
>                         break;
>
>                 default:
> (END)

Hi Leon,

Thanks for your review and for confirming the need for a fix.

Regarding your suggested patch, I believe removing the transition to
TASK_STATE_DRAINED would cause an issue. As seen in the code and comments
for rxe_cleanup_task() and is_done(), the cleanup process waits for the
final TASK_STATE_DRAINED state. If the task remains stuck in DRAINING,
the cleanup loop will never terminate.

My use of cont = 1 was intended as a minimal change. Since this
regression was introduced during the migration from tasklets, restoring
the pre-migration logic seemed like a reasonable approach. An alternative
could be to set the state to TASK_STATE_DRAINED directly inside the
if (!ret) block, and I am open to discussing the best fix.

Regards,
Han