Messages in this thread | | | Subject | Re: [PATCH v2 1/2] tcp/dcpp: Un-pin tw_timer | From | Paolo Abeni <> | Date | Fri, 12 Jan 2024 10:08:07 +0100 |
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Hi,
On Thu, 2023-11-23 at 17:32 +0100, Eric Dumazet wrote: > On Thu, Nov 23, 2023 at 3:34 PM Valentin Schneider <vschneid@redhat.com> wrote: > > So AFAICT, after we go through the hashdance, there's a reference on > > tw_refcnt held by the tw_timer. > > inet_twsk_deschedule_put() can race with arming the timer, but it only > > calls inet_twsk_kill() if the timer > > was already armed & has been deleted, so there's no risk of calling it > > twice... If I got it right :-) > > Again, I think you missed some details. > > I am OOO for a few days, I do not have time to elaborate. > > You will need to properly track active timer by elevating > tw->tw_refcnt, or I guarantee something wrong will happen.
I'm sorry to bring this up again, but I tried to understand what is missing in Valentin's patch and I could not find it.
Direct link to the patch, just in case the thread has been lost: https://lore.kernel.org/lkml/20231115210509.481514-2-vschneid@redhat.com/
The patch raises the initial tw->tw_refcnt to 4, so it tracks (in advance) the reference for the tw_timer. AFAICS the patch is still prone to the race you mentioned on the RFC:
CPU0:
allocates a tw, insert it in hash table
CPU1: finds the TW and removes it (timer cancel does nothing)
CPU0: arms a TW timer, lasting
but I understood such race is acceptable.
Could you please shed some light?
Many thanks,
Paolo
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