netdev - Re: [PATCH v4 net] net: solve a NAPI race

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Date:   Wed, 01 Mar 2017 12:53:08 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     eric.dumazet@...il.com
Cc:     alexander.duyck@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH v4 net] net: solve a NAPI race

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Tue, 28 Feb 2017 10:34:50 -0800

> From: Eric Dumazet <edumazet@...gle.com>
> 
> While playing with mlx4 hardware timestamping of RX packets, I found
> that some packets were received by TCP stack with a ~200 ms delay...
> 
> Since the timestamp was provided by the NIC, and my probe was added
> in tcp_v4_rcv() while in BH handler, I was confident it was not
> a sender issue, or a drop in the network.
> 
> This would happen with a very low probability, but hurting RPC
> workloads.
> 
> A NAPI driver normally arms the IRQ after the napi_complete_done(),
> after NAPI_STATE_SCHED is cleared, so that the hard irq handler can grab
> it.
> 
> Problem is that if another point in the stack grabs NAPI_STATE_SCHED bit
> while IRQ are not disabled, we might have later an IRQ firing and
> finding this bit set, right before napi_complete_done() clears it.
> 
> This can happen with busy polling users, or if gro_flush_timeout is
> used. But some other uses of napi_schedule() in drivers can cause this
> as well.
 ...
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied, thanks Eric.