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

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Message-ID: <20210224180329.306b2207@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 24 Feb 2021 18:03:29 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Alexander Duyck <alexanderduyck@...com>
Cc:     Eric Dumazet <edumazet@...gle.com>, Wei Wang <weiwan@...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 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.