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

[Eric Dumazet <edumazet@google.com>]

On Wed, Nov 15, 2023 at 10:05 PM Valentin Schneider <vschneid@redhat.com> wrote:
>
> 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
>   e02b93124855 ("workqueue: Unbind kworkers before sending them to exit()")
>
> Keep softirqs disabled, but make the timer un-pinned and arm it *after* the
> hashdance.
>
> This introduces the following (non-fatal) race:
>
>   CPU0                        CPU1
>     allocates a tw
>     insert it in hash table
>                                 finds the TW and removes it
>                                 (timer cancel does nothing)
>     arms a TW timer, lasting
>
> This partially reverts
>   ed2e92394589 ("tcp/dccp: fix timewait races in timer handling")
> and
>   ec94c2696f0b ("tcp/dccp: avoid one atomic operation for timewait hashdance")
>
> This also reinstores a comment from
>   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.
>
> Link: https://lore.kernel.org/all/ZPhpfMjSiHVjQkTk@localhost.localdomain/
> Signed-off-by: Valentin Schneider <vschneid@redhat.com>
> ---
>  net/dccp/minisocks.c          | 16 +++++++---------
>  net/ipv4/inet_timewait_sock.c | 20 +++++++++++++++-----
>  net/ipv4/tcp_minisocks.c      | 16 +++++++---------
>  3 files changed, 29 insertions(+), 23 deletions(-)
>
> diff --git a/net/dccp/minisocks.c b/net/dccp/minisocks.c
> index 64d805b27adde..2f0fad4255e36 100644
> --- a/net/dccp/minisocks.c
> +++ b/net/dccp/minisocks.c
> @@ -53,16 +53,14 @@ void dccp_time_wait(struct sock *sk, int state, int timeo)
>                 if (state == DCCP_TIME_WAIT)
>                         timeo = DCCP_TIMEWAIT_LEN;
>
> -               /* tw_timer is pinned, so we need to make sure BH are disabled
> -                * in following section, otherwise timer handler could run before
> -                * we complete the initialization.
> -                */
> -               local_bh_disable();
> -               inet_twsk_schedule(tw, timeo);
> -               /* Linkage updates.
> -                * Note that access to tw after this point is illegal.
> -                */
> +              local_bh_disable();
> +
> +               // Linkage updates
>                 inet_twsk_hashdance(tw, sk, &dccp_hashinfo);
> +               inet_twsk_schedule(tw, timeo);

We could arm a timer there, while another thread/cpu found the TW in
the ehash table.



> +               // Access to tw after this point is illegal.
> +               inet_twsk_put(tw);

This would eventually call inet_twsk_free() while the timer is armed.

I think more work is needed.

Perhaps make sure that a live timer owns a reference on tw->tw_refcnt
(This is not the case atm)