netdev - Re: [PATCH net-next v7 1/3] net: tcp/dccp: prepare for tw

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Date: Wed, 5 Jun 2024 08:57:57 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Florian Westphal <fw@...len.de>
Cc: netdev@...r.kernel.org, Paolo Abeni <pabeni@...hat.com>, 
	"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, vschneid@...hat.com, 
	tglozar@...hat.com, bigeasy@...utronix.de
Subject: Re: [PATCH net-next v7 1/3] net: tcp/dccp: prepare for tw_timer un-pinning

On Tue, Jun 4, 2024 at 4:11 PM Florian Westphal <fw@...len.de> wrote:
>
> From: Valentin Schneider <vschneid@...hat.com>
>
> The TCP timewait timer is proving to be problematic for setups where
> scheduler CPU isolation is achieved at runtime via cpusets (as opposed to
> statically via isolcpus=domains).
>
> What happens there is a CPU goes through tcp_time_wait(), arming the
> time_wait timer, then gets isolated. TCP_TIMEWAIT_LEN later, the timer
> fires, causing interference for the now-isolated CPU. This is conceptually
> similar to the issue described in commit e02b93124855 ("workqueue: Unbind
> kworkers before sending them to exit()")
>
> Move inet_twsk_schedule() to within inet_twsk_hashdance(), with the ehash
> lock held. Expand the lock's critical section from inet_twsk_kill() to
> inet_twsk_deschedule_put(), serializing the scheduling vs descheduling of
> the timer. IOW, this prevents the following race:
>
>                              tcp_time_wait()
>                                inet_twsk_hashdance()
>   inet_twsk_deschedule_put()
>     del_timer_sync()
>                                inet_twsk_schedule()
>
> Thanks to Paolo Abeni for suggesting to leverage the ehash lock.
>
> This also restores a comment from commit ec94c2696f0b ("tcp/dccp: avoid
> one atomic operation for timewait hashdance") as inet_twsk_hashdance() had
> a "Step 1" and "Step 3" comment, but the "Step 2" had gone missing.
>
> inet_twsk_deschedule_put() now acquires the ehash spinlock to synchronize
> with inet_twsk_hashdance_schedule().
>
> To ease possible regression search, actual un-pin is done in next patch.
>
> Link: https://lore.kernel.org/all/ZPhpfMjSiHVjQkTk@localhost.localdomain/
> Signed-off-by: Valentin Schneider <vschneid@...hat.com>
> Signed-off-by: Florian Westphal <fw@...len.de>

Reviewed-by: Eric Dumazet <edumazet@...gle.com>

Thanks.