On 09.04.2021 17:24, Paolo Abeni wrote:
> napi_disable() is subject to an hangup, when the threaded
> mode is enabled and the napi is under heavy traffic.
>
> If the relevant napi has been scheduled and the napi_disable()
> kicks in before the next napi_threaded_wait() completes - so
> that the latter quits due to the napi_disable_pending() condition,
> the existing code leaves the NAPI_STATE_SCHED bit set and the
> napi_disable() loop waiting for such bit will hang.
>
> This patch addresses the issue by dropping the NAPI_STATE_DISABLE
> bit test in napi_thread_wait(). The later napi_threaded_poll()
> iteration will take care of clearing the NAPI_STATE_SCHED.
>
> This also addresses a related problem reported by Jakub:
> before this patch a napi_disable()/napi_enable() pair killed
> the napi thread, effectively disabling the threaded mode.
> On the patched kernel napi_disable() simply stops scheduling
> the relevant thread.
>
> v1 -> v2:
> - let the main napi_thread_poll() loop clear the SCHED bit
>
> Reported-by: Jakub Kicinski <k...@kernel.org>
> Fixes: 29863d41bb6e ("net: implement threaded-able napi poll loop support")
> Signed-off-by: Paolo Abeni <pab...@redhat.com>
> ---
> net/core/dev.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 0f72ff5d34ba..af8c1ea040b9 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -6992,7 +6992,7 @@ static int napi_thread_wait(struct napi_struct *napi)
>
> set_current_state(TASK_INTERRUPTIBLE);
>
> - while (!kthread_should_stop() && !napi_disable_pending(napi)) {
> + while (!kthread_should_stop()) {
> /* Testing SCHED_THREADED bit here to make sure the current
> * kthread owns this napi and could poll on this napi.
> * Testing SCHED bit is not enough because SCHED bit might be
> @@ -7010,6 +7010,7 @@ static int napi_thread_wait(struct napi_struct *napi)
> set_current_state(TASK_INTERRUPTIBLE);
> }
> __set_current_state(TASK_RUNNING);
> +
> return -1;
> }
>
>
Unrelated to the actual issue:
I wonder why -1 is returned, that's unusual in kernel. The return value
is used as bool, so why not return a bool?
