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Message-ID: <CANn89iLtzzqHhFtq196AnAer6YoUjQKxHz2_zsqbiavnZAqUjQ@mail.gmail.com> Date: Wed, 31 May 2023 06:13:02 +0200 From: Eric Dumazet <edumazet@...gle.com> To: Jason Xing <kerneljasonxing@...il.com> Cc: fuyuanli <fuyuanli@...iglobal.com>, "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Neal Cardwell <ncardwell@...gle.com>, ycheng <ycheng@...gle.com>, toke <toke@...e.dk>, netdev@...r.kernel.org, Weiping Zhang <zhangweiping@...iglobal.com>, Tio Zhang <tiozhang@...iglobal.com> Subject: Re: [PATCH net v2] tcp: fix mishandling when the sack compression is deferred On Wed, May 31, 2023 at 5:50 AM Jason Xing <kerneljasonxing@...il.com> wrote: > > On Tue, May 30, 2023 at 10:51 PM Eric Dumazet <edumazet@...gle.com> wrote: > > > > On Tue, May 30, 2023 at 4:32 PM Jason Xing <kerneljasonxing@...il.com> wrote: > > > > > > I'm confused. You said in the previous email: > > > "As a bonus, no need to send one patch for net, and another in net-next, > > > trying to 'fix' issues that should have been fixed cleanly in a single patch." > > > > > > So we added "introducing ICSK_ACK_TIMER flag for sack compression" to > > > fix them on top of the patch you suggested. > > > > > > I can remove the Suggested-by label. For now, I do care about your > > > opinion on the current patch. > > > > > > Well...should I give up introducing that flag and then leave that > > > 'issue' behind? :S > > > > Please let the fix go alone. > > > > Then I will look at your patch, but honestly I fail to see the _reason_ for it. > > > > In case you missed it, tcp_event_ack_sent() calls > > inet_csk_clear_xmit_timer(sk, ICSK_TIME_DACK); > > Hello Eric, > > Sorry, I didn't explain that 'issue' well last night. Let me try it once more: > > In the tcp_event_ack_sent(), since we're going to transmit data with > ack header, we should cancel those timers which could start before to > avoid sending useless/redundant acks. Right? > > But what if the timer, say, icsk_delack_timer, was triggered before > and had to postpone it in the release cb phrase because currently > socket (in the tcp sending process) has owned its @owned > field(sk->sk_lock.owned == 1). > > We could avoid sending extra useless ack by removing the > ICSK_ACK_TIMER flag to stop sending an ack in > tcp_delack_timer_handler(). > > In the current logic, see in the tcp_event_ack_sent(): > 1) hrtimer_try_to_cancel(&tp->compressed_ack_timer) > 2) sk_stop_timer(sk, &icsk->icsk_delack_timer) > Those two statements can prevent the timers from sending a useless ack > but cannot prevent sending a useless ack in the deferred process. > > Does it make any sense? Like I said, it's not a bug, but more like an > improvement. Your patch adds a bug. An skb allocation can fail, and ACK would not be sent. Timer handlers are not canceled in TCP stack. We do not call sk_stop_timer() because include/net/inet_connection_sock.h says /* Cancel timers, when they are not required. */ #undef INET_CSK_CLEAR_TIMERS So claiming the following is nonsense: <quote> 2) sk_stop_timer(sk, &icsk->icsk_delack_timer) Those two statements can prevent ... </quote> We do not send extra ACK, because icsk->icsk_ack.pending (or icsk->icsk_pending) is cleared in inet_csk_clear_xmit_timer() This clearing is happening already at strategic places. When tcp_delack_timer_handler() is finally run (when owning socket lock), it will return early if icsk->icsk_ack.pending was already cleared. hrtimer_try_to_cancel(&tp->compressed_ack_timer) has to be called because we rely on the hrtimer status (hrtimer_is_queued()) in __tcp_ack_snd_check()
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