Date: Mon, 12 Apr 2010 12:56:12 -0400 From: John Baldwin <jhb@freebsd.org> To: Jack Vogel <jfvogel@gmail.com> Cc: pyunyh@gmail.com, freebsd-current@freebsd.org, Mike Tancsa <mike@sentex.net> Subject: Re: LOR on em in HEAD ( was Re: em driver regression Message-ID: <201004121256.12161.jhb@freebsd.org> In-Reply-To: <z2l2a41acea1004120926k728fe54cz4e29503ed9ab42c2@mail.gmail.com> References: <201004081313.o38DD4JM041821@lava.sentex.ca> <201004121052.42350.jhb@freebsd.org> <z2l2a41acea1004120926k728fe54cz4e29503ed9ab42c2@mail.gmail.com>
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On Monday 12 April 2010 12:26:06 pm Jack Vogel wrote: > On Mon, Apr 12, 2010 at 7:52 AM, John Baldwin <jhb@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 > > UDP > > 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 > > LOR > > 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? Well, while that might quiet the LOR, I suspect it would be masking another problem that is the "real" LOR. -- John Baldwin
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