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

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Date:   Fri, 2 Nov 2018 16:15:52 -0600
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Saeed Mahameed <saeedm@...lanox.com>
Cc:     "eric.dumazet@...il.com" <eric.dumazet@...il.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "arnd@...db.de" <arnd@...db.de>,
        "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 Fri, Nov 02, 2018 at 10:07:02PM +0000, Saeed Mahameed 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.

That was my guess when I saw this.. the confusing bit is why is there
s and temp, why not just s?

> BTW memcpy itself is not thread safe.

At least on 64 bit memcpy will do > 8 byte stores when copying so on
most architectures it will cause individual new or old u64 to be
returned and not a mess..

32 bit will always make a mess.

If the stats don't update that often then kmalloc'ing a new buffer and
RCU'ing it into view might be a reasonable alternative to this?

Jason