Date: Mon, 12 Apr 2010 09:26:06 -0700 From: Jack Vogel <jfvogel@gmail.com> To: John Baldwin <jhb@freebsd.org> 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: <z2l2a41acea1004120926k728fe54cz4e29503ed9ab42c2@mail.gmail.com> In-Reply-To: <201004121052.42350.jhb@freebsd.org> References: <201004081313.o38DD4JM041821@lava.sentex.ca> <201004091900.o39J0b0k051687@lava.sentex.ca> <z2r2a41acea1004091209maca81e47td7f03d7c343b3ec9@mail.gmail.com> <201004121052.42350.jhb@freebsd.org>
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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? Jack
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