Am?rico Wang <xiyou.wangcong at gmail.com> writes: > On Mon, Oct 24, 2011 at 11:14 PM, Eric W. Biederman > <ebiederm at xmission.com> wrote: >> Michael Holzheu <holzheu at linux.vnet.ibm.com> writes: >> >>> Hello Vivek, >>> >>> In our tests we ran into the following scenario: >>> >>> Two CPUs have called panic at the same time. The first CPU called >>> crash_kexec() and the second CPU called smp_send_stop() in panic() >>> before crash_kexec() finished on the first CPU. So the second CPU >>> stopped the first CPU and therefore kdump failed. >>> >>> 1st CPU: >>> panic()->crash_kexec()->mutex_trylock(&kexec_mutex)-> do kdump >>> >>> 2nd CPU: >>> panic()->crash_kexec()->kexec_mutex already held by 1st CPU >>> ? ? ? ?->smp_send_stop()-> stop CPU 1 (stop kdump) >>> >>> How should we fix this problem? One possibility could be to do >>> smp_send_stop() before we call crash_kexec(). >>> >>> What do you think? >> >> smp_send_stop is insufficiently reliable to be used before crash_kexec. >> >> My first reaction would be to test oops_in_progress and wait until >> oops_in_progress == 1 before calling smp_send_stop. >> > > +1 > > One of my colleague mentioned the same problem with me inside > RH, given the fact that the race condition window is small, it would > not be easy to reproduce this scenario. As for reproducing it I have a hunch you could hack up something horrible with smp_call_function and kprobes. On a little more reflection we can't wait until oops_in_progress goes to 1 before calling smp_send_stop. Because if crash_kexec is not involved nothing we will never call smp_send_stop. So my second thought is to introduce another atomic variable panic_in_progress, visible only in panic. The cpu that sets increments panic_in_progress can call smp_send_stop. The rest of the cpus can just go into a busy wait. That should stop nasty fights about who is going to come out of smp_send_stop first. Eric
