Re: Fw: memory corruption in tcp bind hash buckets on SMP?

[Raghu Angadi]

inverted order of insertion into the lists in tw_hashdance() is probably
cleaner fix than inverted order of removal.. (that is.. if you dont like "if
(tb) put();" or double refcnting :-)

--- Raghu Angadi <[e-mail]> wrote:
> 
> --- [e-mail] wrote:
> > Hello!
> > 
> > > I think his analysis is alright but he patch is questionable.
> > 
> > Yes. "if (tb) tcp_tw_put(tw)" cannot be right, no doubts.
> 
> In timewait_kill(), tb is set only _after_ it has been removed from the
> established (if it exist there). In _hashdance() tw is inserted into
> established _before_ it is inserted into bindhash. So the above is one way
> of
> saying do tw_put() when it has been deleted from _both_ the lists.
> Also note that patch removes "if (!tw->pprev) return;" thingy. Infact this
> return sort of implied you never thought tw could be deleted from ehash and
> not from bind hash.
> 
> > Seems, it is enough to remove from bind hash _before_ established.
> > 
> > The idea was that bind hash is pure slave of another state, so that
> > it need not refcounting at all. Note that adding the second increment
> > does not help: when we verify that leakage (the situation, when
> > bucket is in bind hash, but has no timer running) is impossible
> > we immediately arrive to elimination of the refcount.
> > 
> > Raghu, could you check the variant with inverted order of removal?
> > Do you see holes? From my side... I need to think more. :-)
> 
> Just the inteverted order of removal will fix _this_ perticular case.
> But we still end up doing tw_put() in timewait_kill() even though tw is
> still
> in the bind list (just got inserted on the other processor). This seems
> conceptually incorrect or confusing. The refcnt increment in tw_hashdance()
> is for these two lists and tw_put() in timewait_kill() should correspond to
> the deletion from _both_ the lists. 
>  
> If you want to avoid timewait_kill() getting called twice altogether.. then
> its a different issue.
> 
> Raghu.
> 
> > Alexey
> 
> 
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