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From: Julian Elischer <julian@elischer.org>
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Cc: Attilio Rao <attilio@freebsd.org>, FreeBSD Current <current@freebsd.org>
Subject: Re: crash in tty code in 6.1.. fixed since?
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>>> tp_session is NULL but it shouldn't have been able to have run that line 
>>> (line 1659) if it had tested NULL 2 lines before..
>>> this suggests a locking problem..
>> I think it has been fixed some months ago IIRC.
>> The problem here, should be that if sx_slock() let thread sleep, Giant 
>> is released before to sleep and tp->t_session can be accessed in racy way.
>> Another nice side-effect about having tty Giant :)
> 
> Yes, it seems that rev. 1.267 fixed it and was MFCed as rev. 1.228.2.6.
> There was a lot of commits around this one, it may be safer to update to
> RELENG_6.

That's not an option on a product. 
6.1 has passed all sorts of QA benchmarks etc. and while we will upgrade,
it will probably happen next year.