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Date:   Thu, 27 Oct 2022 16:34:53 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Stephen Boyd <sboyd@kernel.org>,
        Guenter Roeck <linux@roeck-us.net>,
        Jesse Brandeburg <jesse.brandeburg@intel.com>,
        Tony Nguyen <anthony.l.nguyen@intel.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Mirko Lindner <mlindner@marvell.com>,
        Stephen Hemminger <stephen@networkplumber.org>,
        Martin KaFai Lau <martin.lau@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Kuniyuki Iwashima <kuniyu@amazon.com>,
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Subject: Re: [RFC][PATCH v2 19/31] timers: net: Use del_timer_shutdown()
 before freeing timer
Message-ID: <20221027163453.383bbf8e@gandalf.local.home>
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On Thu, 27 Oct 2022 13:15:23 -0700
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Thu, Oct 27, 2022 at 12:55 PM Steven Rostedt <rostedt@goodmis.org> wrote:
> >
> > I think we need to update this code to squeeze in a del_timer_shutdown() to
> > make sure that the timers are never restarted.  
> 
> So the reason the networking code does this is that it can't just do
> the old 'sync()' thing, the timers are deleted in contexts where that
> isn't valid.
> 
> Which is also afaik why the networking code does that whole "timer
> implies a refcount to the socket" and then does the
> 
>     if (del_timer(timer))
>            sock_put()
> 
> thing (ie if the del_timer failed - possibly because it was already
> running - you leave the refcount alone).

OK, so the above is assuming that the timer is always active, and
del_timer() returns if it successfully removed it (where it can call
sock_put()), but if del_timer() returns 0, that means the timer is
currently running (or about to be), so it doesn't call sock_put().

> 
> So the networking code cannot do the del_timer_shutdown() for the same
> reason it cannot do the del_timer_sync(): it can't afford to wait for
> the timer to stop running.
> 
> I suspect it needs something like a new "del_timer_shutdown_async()"
> that isn't synchronous, but does that
> 
>  - acts as del_timer in that it doesn't wait, and returns a success if
> it could just remove the pending case
> 
>  - does that "mark timer for shutdown" in that success case
> 
> or something similar.
>

What about del_timer_try_shutdown(), that if it removes the timer, it sets
the function to NULL (making it equivalent to a successful shutdown),
otherwise it does nothing. Allowing the the timer to be rearmed.

I think this would work in this case.

-- Steve