netdev - Re: [PATCH v2 1/2] tcp/dcpp: Un-pin tw

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Date: Fri, 12 Jan 2024 10:08:07 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Eric Dumazet <edumazet@...gle.com>, Valentin Schneider
 <vschneid@...hat.com>
Cc: dccp@...r.kernel.org, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org,  linux-rt-users@...r.kernel.org, "David S.
 Miller" <davem@...emloft.net>,  Jakub Kicinski <kuba@...nel.org>, David
 Ahern <dsahern@...nel.org>, Juri Lelli <juri.lelli@...hat.com>, Tomas
 Glozar <tglozar@...hat.com>, Sebastian Andrzej Siewior
 <bigeasy@...utronix.de>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 1/2] tcp/dcpp: Un-pin tw_timer

Hi,

On Thu, 2023-11-23 at 17:32 +0100, Eric Dumazet wrote:
> On Thu, Nov 23, 2023 at 3:34 PM Valentin Schneider <vschneid@...hat.com> wrote:
> > So AFAICT, after we go through the hashdance, there's a reference on
> > tw_refcnt held by the tw_timer.
> > inet_twsk_deschedule_put() can race with arming the timer, but it only
> > calls inet_twsk_kill() if the timer
> > was already armed & has been deleted, so there's no risk of calling it
> > twice... If I got it right :-)
> 
> Again, I think you missed some details.
> 
> I am OOO for a few days, I do not have time to elaborate.
> 
> You will need to properly track active timer by elevating
> tw->tw_refcnt, or I guarantee something wrong will happen.

I'm sorry to bring this up again, but I tried to understand what is
missing in Valentin's patch and I could not find it.

Direct link to the patch, just in case the thread has been lost:
https://lore.kernel.org/lkml/20231115210509.481514-2-vschneid@redhat.com/

The patch raises the initial tw->tw_refcnt to 4, so it tracks (in
advance) the reference for the tw_timer. AFAICS the patch is still
prone to the race you mentioned on the RFC:

CPU0:

   allocates a tw, insert it in hash table

CPU1:
   finds the TW and removes it (timer cancel does nothing)

CPU0:
   arms a TW timer, lasting

but I understood such race is acceptable.

Could you please shed some light? 

Many thanks,

Paolo