netdev - Re: [PATCH net] net: fix hangup on napi

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Message-ID: <20210331184137.129fc965@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 31 Mar 2021 18:41:37 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
        Wei Wang <weiwan@...gle.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH net] net: fix hangup on napi_disable for threaded napi

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 <pabeni@...hat.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()?