On Thu, 1 Apr 2021 00:46:18 +0200 Paolo Abeni wrote:
> I hit an hangup on napi_disable(), 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.
>
> Address the issue explicitly clearing the SCHED_BIT on napi_thread
> termination, if the thread is owns the napi.
>
> Fixes: 29863d41bb6e ("net: implement threaded-able napi poll loop support")
> Signed-off-by: Paolo Abeni <pab...@redhat.com>
> ---
> net/core/dev.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index b4c67a5be606d..e2e716ba027b8 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -7059,6 +7059,14 @@ static int napi_thread_wait(struct napi_struct *napi)
> set_current_state(TASK_INTERRUPTIBLE);
> }
> __set_current_state(TASK_RUNNING);
> +
> + /* if the thread owns this napi, and the napi itself has been disabled
> + * in-between napi_schedule() and the above napi_disable_pending()
> + * check, we need to clear the SCHED bit here, or napi_disable
> + * will hang waiting for such bit being cleared
> + */
> + if (test_bit(NAPI_STATE_SCHED_THREADED, &napi->state) || woken)
> + clear_bit(NAPI_STATE_SCHED, &napi->state);
Not sure this covers 100% of the cases. We depend on the ability to go
through schedule() "unnecessarily" when the napi gets scheduled after
we go into TASK_INTERRUPTIBLE.
If we just check woken outside of the loop it may be false even though
we got a "wake event".
Looking closer now I don't really understand where we ended up with
disable handling :S Seems like the thread exits on napi_disable(),
but is reaped by netif_napi_del(). Some drivers (*cough* nfp) will
go napi_disable() -> napi_enable()... and that will break.
Am I missing something?
Should we not stay in the wait loop on napi_disable()?
