linux-kernel - Re: [PATCH] netlink: introduce netlink poll to resolve fast return issue

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Message-ID: <20231110110002.7279f895@kernel.org>
Date:   Fri, 10 Nov 2023 11:00:02 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     "Jong eon Park" <jongeon.park@...sung.com>
Cc:     "'Paolo Abeni'" <pabeni@...hat.com>,
        "'David S. Miller'" <davem@...emloft.net>,
        "'Eric Dumazet'" <edumazet@...gle.com>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        "'Dong ha Kang'" <dongha7.kang@...sung.com>
Subject: Re: [PATCH] netlink: introduce netlink poll to resolve fast return
 issue

On Fri, 10 Nov 2023 23:54:48 +0900 Jong eon Park wrote:
> Interestingly, in this issue, even though netlink overrun frequently 
> happened and caused POLLERRs, the user was managing it well through 
> POLLIN and 'recv' function without a specific POLLERR handler. 
> However, in the current situation, rcv queue is already empty and 
> NETLINK_S_CONGESTED flag prevents any more incoming packets. This makes 
> it impossible for the user to call 'recv'.
> 
> This "congested" situation is a bit ambiguous. The queue is empty, yet 
> 'congested' remains. This means kernel can no longer deliver uevents 
> despite the empty queue, and it lead to the persistent 'congested' status.
> 
> The reason for the difference in netlink lies in the NETLINK_S_CONGESTED 
> flag. If it were UDP, upon seeing the empty queue, it might have kept 
> pushing the received packets into the queue (making possible to call 
> 'recv').

I see, please add a comment saying that NETLINK_S_CONGESTED prevents
new skbs from being queued before the new test in netlink_poll().

Please repost next week (i.e. after the merge window) with subject
tagged [PATCH net-next v2].