Messages in this thread | | | Date | Thu, 1 Mar 2012 16:24:18 -0600 | From | Jason Wessel <> | Subject | Re: [PATCHv3 1/3] NETPOLL: Extend rx_hook support. |
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On 03/01/2012 03:04 PM, Andrei Warkentin wrote: > ----- Original Message ----- >> From: "Andrei Warkentin" <awarkentin@vmware.com> >> To: "Jason Wessel" <jason.wessel@windriver.com> >> Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, "Andrei Warkentin" <andreiw@vmware.com>, >> kgdb-bugreport@lists.sourceforge.net, "Matt Mackall" <mpm@selenic.com>, "Andrei Warkentin" >> <andrey.warkentin@gmail.com> >> Sent: Tuesday, February 28, 2012 12:43:52 PM >> Subject: Re: [PATCHv3 1/3] NETPOLL: Extend rx_hook support. >> >>> All that netpoll_poll() did was to call netpoll_poll_dev(). I have >>> not yet looked at the differences between kgdboe and the netkdb >>> code >>> you proposed but I would have suspected it also falls victim to the >>> ethernet preemption problem which prevented kgdboe from ever being >>> considered for a mainline merge. Certainly there are ways to fix >>> this >>> problem but most involved changes to scheduling, core net code, or >>> substantial driver specific changes. >>> >> I see, I read up on the issues w.r.t. preemption. Could this be >> worked >> around by modifiying affected drivers to bypass locking if they are >> used in KDB context? Make some accessor netdev-specific lock/unlocks >> that won't do anything if running in KDB context. >> >> > By the way, is there a good way to repro the preemption case? Hopefully this doesn't > involve some crazy hardware...
I have several cases which will usually hang the machine fairly quickly, but they all involve using gdb and a target using SMP. Most often it is as simple as this:
* Use an SMP system with with at least 2 cores * Start two threads rapidly running some processes while [ 1 ] ; do date > /dev/null ; done & while [ 1 ] ; do date > /dev/null ; done & * Connect with gdb to kgdb and set a breakpoint at do_fork Now do "c" Now do "c 1000"
Generally the system will hang long before you get 1000 breakpoints hit and it will be a condition where there is a lock needed to create an skb, or the ethernet driver is preempted or some part of the network stack is preempted (or holding a lock) on the non master cpu.
There is another condition that is hard to catch that involves a task migrating from one cpu to the next, but we'll stick to the simple test case I described above for now.
I did have a question, because it seems you were using qemu / kvm. I have a number of test cases that use kvm, but the netkkgdb does not seem to work with the nc. My question is how am I supposed to actually use the netkgdb?
Here is what I observe on the target system:
insmod netkgdb.ko netkgdb=@/,@10.0.2.2/ echo g > /proc/sysrq-trigger
On my host system: nc.traditional -l -u -p 7777
I will type help, and then the netkgdb is toast. It doesn't seem to respond anymore.
Jason.
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