linux-kernel - Re: [RFC][PATCH v2 19/31] timers: net: Use del_timer

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Date:   Sun, 30 Oct 2022 18:22:03 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Tony Nguyen <anthony.l.nguyen@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Mirko Lindner <mlindner@...vell.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Martin KaFai Lau <martin.lau@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Kuniyuki Iwashima <kuniyu@...zon.com>,
        Pavel Begunkov <asml.silence@...il.com>,
        Menglong Dong <imagedong@...cent.com>,
        linux-usb@...r.kernel.org, linux-wireless@...r.kernel.org,
        bridge@...ts.linux-foundation.org, netfilter-devel@...r.kernel.org,
        coreteam@...filter.org, lvs-devel@...r.kernel.org,
        linux-afs@...ts.infradead.org, linux-nfs@...r.kernel.org,
        tipc-discussion@...ts.sourceforge.net
Subject: Re: [RFC][PATCH v2 19/31] timers: net: Use del_timer_shutdown()
 before freeing timer

On Fri, 2022-10-28 at 15:46 -0700, Jakub Kicinski wrote:
> On Fri, 28 Oct 2022 18:31:49 -0400 Steven Rostedt wrote:
> > Could someone from networking confirm (or deny) that the timer being
> > removed in sk_stop_timer() will no longer be used even if del_timer()
> > returns false?
> > 
> > net/core/sock.c:
> > 
> > void sk_stop_timer(struct sock *sk, struct timer_list* timer)
> > {
> > 	if (del_timer(timer))
> > 		__sock_put(sk);
> > }
> > 
> > If this is the case, then I'll add the following interface:
> > 
> >    del_timer_sync_shutdown() // the common case which syncs
> > 
> >    del_timer_shutdown() // the uncommon case, that returns immediately
> >                         // used for those cases that add extra code to
> >                         // handle it, like sk_stop_timer()
> 
> Sorry too many bugs at once :)
> 
> FWIW Paolo was saying privately earlier today that he spotted some cases
> of reuse, he gave an example of ccid2_hc_tx_packet_recv()

For the records, there are other cases, e.g. after sk_stop_timer() in 
clear_3rdack_retransmission() (mptcp code) the timer can be-rearmed
without re-initializing. I *think* there are more of such use in the 
in ax25/rose code.

> So we can't convert all cases of sk_stop_timer() in one fell swoop :(

On the positive side, I think converting the sk_stop_timer in 
inet_csk_clear_xmit_timers() should be safe and should cover the issue
reported by Guenter

Cheers,

Paolo