linux-kernel - Re: [PATCH] net/mlx5e: fix high stack usage

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Date:   Fri, 2 Nov 2018 23:15:11 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Saeed Mahameed <saeedm@...lanox.com>
Cc:     "eric.dumazet@...il.com" <eric.dumazet@...il.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "leon@...nel.org" <leon@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Moshe Shemesh <moshe@...lanox.com>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        Boris Pismenny <borisp@...lanox.com>,
        Tariq Toukan <tariqt@...lanox.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        Eran Ben Elisha <eranbe@...lanox.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Ilya Lesokhin <ilyal@...lanox.com>,
        Kamal Heib <kamalh@...lanox.com>
Subject: Re: [PATCH] net/mlx5e: fix high stack usage

On 11/2/18, Saeed Mahameed <saeedm@...lanox.com> wrote:
> On Fri, 2018-11-02 at 14:39 -0700, Eric Dumazet wrote:
>>
>> On 11/02/2018 02:05 PM, Saeed Mahameed wrote:
>>
>> > temp will be mem copied to priv->stats.sw at the end,
>> > memcpy(&priv->stats.sw, &s, sizeof(s));
>> >
>> > one other way to solve this as suggested by Andrew, is to get rid
>> > of
>> > the temp var and make it point directly to priv->stats.sw
>> >
>>
>> What about concurrency ?
>>
>> This temp variable is there to make sure concurrent readers of stats
>> might
>> not see mangle data (because another 'reader' just did a memset() and
>> is doing the folding)
>>
>>
>> mlx5e_get_stats() can definitely be run at the same time by multiple
>> threads.
>>
>
> hmm, you are right, i was thinking that mlx5e_get_Stats will trigger a
> work to update stats and grab the state_lock, but for sw stats this is
> not the case it is done in place.
>
> BTW memcpy itself is not thread safe.

Before commit 6c63efe4cfab ("net/mlx5e: Remove redundant active_channels
indication"), there was a read_lock() in the function apparently intended to
made it thread safe. This got removed with the comment

commit 6c63efe4cfabf230a8ed4b1d880249875ffdac13
Author: Eran Ben Elisha <eranbe@...lanox.com>
Date:   Tue May 29 11:06:31 2018 +0300

    net/mlx5e: Remove redundant active_channels indication

    Now, when all channels stats are saved regardless of the channel's state
    {open, closed}, we can safely remove this indication and the stats spin
    lock which protects it.

    Fixes: 76c3810bade3 ("net/mlx5e: Avoid reset netdev stats on
configuration changes")

I don't really understand the reasoning, but maybe we can remove
the memcpy() if the code is thread safe, or we need the lock back if it's not.

         Arnd