linux-kernel - Re: [PATCH v5 0/2] tcp/dcpp: Un-pin tw

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Date: Tue, 16 Apr 2024 17:01:47 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: 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>, Paolo Abeni <pabeni@...hat.com>, mleitner@...hat.com, 
	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 v5 0/2] tcp/dcpp: Un-pin tw_timer

On Mon, Apr 15, 2024 at 4:33 PM Valentin Schneider <vschneid@...hatcom> wrote:
>
> On 15/04/24 14:35, Eric Dumazet wrote:
> > On Mon, Apr 15, 2024 at 1:34 PM Valentin Schneider <vschneid@...hat.com> wrote:
> >>
> >> Hi,
> >>
> >> This is v5 of the series where the tw_timer is un-pinned to get rid of
> >> interferences in isolated CPUs setups.
> >>
> >> The first patch is a new one stemming from Jakub's bug reported. It's there
> >> mainly to make the reviewing a bit easier, but as it changes behaviour it should
> >> be squashed with the second one.
> >>
> >> Revisions
> >> =========
> >>
> >> v4 -> v5
> >> ++++++++
> >>
> >> o Rebased against latest Linus' tree
> >> o Converted tw_timer into a delayed work following Jakub's bug report on v4
> >>   http://lore.kernel.org/r/20240411100536.224fa1e7@kernel.org
> >
> > What was the issue again ?
> >
> > Please explain precisely why it was fundamentally tied to the use of
> > timers (and this was not possible to fix the issue without
> > adding work queues and more dependencies to TCP stack)
>
> In v4 I added the use of the ehash lock to serialize arming the timewait
> timer vs destroying it (inet_twsk_schedule() vs inet_twsk_deschedule_put()).
>
> Unfortunately, holding a lock both in a timer callback and in the context
> in which it is destroyed is invalid. AIUI the issue is as follows:
>
>   CPUx                        CPUy
>   spin_lock(foo);
>                               <timer fires>
>                               call_timer_fn()
>                                 spin_lock(foo) // blocks
>   timer_shutdown_sync()
>     __timer_delete_sync()
>       __try_to_del_timer_sync() // looped as long as timer is running
>                        <deadlock>
>
> In our case, we had in v4:
>
>   inet_twsk_deschedule_put()
>     spin_lock(ehash_lock);
>                                           tw_timer_handler()
>                                             inet_twsk_kill()
>                                               spin_lock(ehash_lock);
>                                               __inet_twsk_kill();
>     timer_shutdown_sync(&tw->tw_timer);
>
> The fix here is to move the timer deletion to a non-timer
> context. Workqueues fit the bill, and as the tw_timer_handler() would just queue
> a work item, I converted it to a delayed_work.

I do not like this delayed work approach.

Adding more dependencies to the TCP stack is not very nice from a
maintainer point of view.

Why couldn't you call timer_shutdown_sync() before grabbing the lock ?