netdev - Re: [PATCH net-next] net: enetc: fix TX ring interrupt storm

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Date:   Sat, 10 Apr 2021 00:29:23 +0300
From:   Claudiu Manoil <claudiu.manoil@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Cc:     Alex Marginean <alexandru.marginean@....com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Yangbo Lu <yangbo.lu@....com>,
        Vladimir Oltean <vladimir.oltean@....com>
Subject: Re: [PATCH net-next] net: enetc: fix TX ring interrupt storm

On 09.04.2021 22:27, Vladimir Oltean wrote:
> From: Vladimir Oltean <vladimir.oltean@....com>
> 
> The blamed commit introduced a bit in the TX software buffer descriptor
> structure for determining whether a BD is final or not; we rearm the TX
> interrupt vector for every frame (hence final BD) transmitted.
> 
> But there is a problem with the patch: it replaced a condition whose
> expression is a bool which was evaluated at the beginning of the "while"
> loop with a bool expression that is evaluated on the spot: tx_swbd->is_eof.
> 
> The problem with the latter expression is that the tx_swbd has already
> been incremented at that stage, so the tx_swbd->is_eof check is in fact
> with the _next_ software BD. Which is _not_ final.
> 
> The effect is that the CPU is in 100% load with ksoftirqd because it
> does not acknowledge the TX interrupt, so the handler keeps getting
> called again and again.
> 
> The fix is to restore the code structure, and keep the local bool is_eof
> variable, just to assign it the tx_swbd->is_eof value instead of
> !!tx_swbd->skb.
> 
> Fixes: d504498d2eb3 ("net: enetc: add a dedicated is_eof bit in the TX software BD")
> Reported-by: Alex Marginean <alexandru.marginean@....com>
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>

Reviewed-by: Claudiu Manoil <claudiu.manoil@....com>