netdev - Re: [PATCH net] net: fix race between napi kthread mode and busy poll

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Date:   Wed, 24 Feb 2021 18:31:55 -0800
From:   Wei Wang <weiwan@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Alexander Duyck <alexanderduyck@...com>,
        Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>, Paolo Abeni <pabeni@...hat.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Martin Zaharinov <micron10@...il.com>
Subject: Re: [PATCH net] net: fix race between napi kthread mode and busy poll

On Wed, Feb 24, 2021 at 6:03 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Thu, 25 Feb 2021 01:22:08 +0000 Alexander Duyck wrote:
> > Yeah, that was the patch Wei had done earlier. Eric complained about the extra set_bit atomic operation in the threaded path. That is when I came up with the idea of just adding a bit to the busy poll logic so that the only extra cost in the threaded path was having to check 2 bits instead of 1.
>
> Maybe we can set the bit only if the thread is running? When thread
> comes out of schedule() it can be sure that it has an NAPI to service.
> But when it enters napi_thread_wait() and before it hits schedule()
> it must be careful to make sure the NAPI is still (or already in the
> very first run after creation) owned by it.

Are you suggesting setting the SCHED_THREAD bit in napi_thread_wait()
somewhere instead of in ____napi_schedule() as you previously plotted?
What does it help? I think if we have to do an extra set_bit(), it
seems cleaner to set it in ____napi_schedule(). This would solve the
warning issue as well.