From: Paolo Abeni <pabeni@redhat.com>
To: Eric Dumazet <edumazet@google.com>,
Valentin Schneider <vschneid@redhat.com>
Cc: dccp@vger.kernel.org, netdev@vger.kernel.org,
linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org,
"David S. Miller" <davem@davemloft.net>,
Jakub Kicinski <kuba@kernel.org>,
David Ahern <dsahern@kernel.org>,
Juri Lelli <juri.lelli@redhat.com>,
Tomas Glozar <tglozar@redhat.com>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v2 1/2] tcp/dcpp: Un-pin tw_timer
Date: Fri, 12 Jan 2024 10:08:07 +0100 [thread overview]
Message-ID: <9619119e125b0e30461e147f5654001397eb0fe6.camel@redhat.com> (raw)
In-Reply-To: <CANn89iKRSKz0e8v+Z-UsKGs4fQWDt6eTAw71VENbSmfkEicTPA@mail.gmail.com>
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@redhat.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
next prev parent reply other threads:[~2024-01-12 9:08 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-15 21:05 [PATCH v2 0/2] tcp/dcpp: tw_timer tweaks for nohz_full and PREEMPT_RT Valentin Schneider
2023-11-15 21:05 ` [PATCH v2 1/2] tcp/dcpp: Un-pin tw_timer Valentin Schneider
2023-11-20 17:55 ` Eric Dumazet
2023-11-23 14:34 ` Valentin Schneider
2023-11-23 16:32 ` Eric Dumazet
2023-11-23 16:45 ` Valentin Schneider
2023-12-08 14:47 ` Valentin Schneider
2024-01-12 9:08 ` Paolo Abeni [this message]
2023-11-15 21:05 ` [PATCH v2 2/2] tcp/dcpp: Don't disable bh around timewait_sock initialization Valentin Schneider
2023-11-20 15:23 ` kernel test robot
2023-11-23 15:26 ` Valentin Schneider
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