LOR on em in HEAD ( was Re: em driver regression

Jack Vogel jfvogel at gmail.com
Mon Apr 12 16:26:08 UTC 2010

On Mon, Apr 12, 2010 at 7:52 AM, John Baldwin <jhb at freebsd.org> wrote:

> On Friday 09 April 2010 3:09:24 pm Jack Vogel wrote:
> > Someone else also pointed this out. I'm dubious about its claim.
> > This happens because there is an RX lock taken in rxeof, its held
> > thru the call into the stack, it then encounters another lock there
> > and hence this complaint. I've had the RX hold as it is for a long
> > while and would rather not have to give it up, can someone look
> > at it and advise?
> I've seen it happen with igb.  I suspect it is a transitive lock order.
>  That
> is, you probably never have the UDP lock acquired before an em/igb RX lock.
> However, if you have an em/igb adapter TX lock held when you acquire an
> em/igb
> RX lock in one place, and in if_start() you acquire the TX lock while the
> lock is held, that can trigger the LOR.  Specifically, those two paths
> would
> give you these two orders:
> TX -> RX
> UDP -> TX
> which implies the order
> UDP -> RX
> (lock order relationsips are transitive, just like a > b and b > c implies
> a > c).
> However, I haven't been able to track down what the raw orders are that
> might
> lead to this transitive order.  Attilio added some sysctls to dump all the
> raw
> lock orders in one of the debug.witness sysctls.  You can also try
> hardcoding
> the 'RX -> UDP' order using WITNESS_DEFINEORDER() before any of the em/igb
> RX/TX locks are acquired to see what different LOR is triggered.  If that
> looks valid then you can keep hardcoding valid orders until you find the
> invalid one.
> Do you think releasing the RX lock before the stack entry would get rid of
the problem?

Other ideas?


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