On 11/03/15 07:59, Mark Johnston wrote:
On Tue, Mar 10, 2015 at 02:10:09PM -0400, John Baldwin wrote:
Often loops using libkvm are due to programs using libkvm are trying to read
kernel data structures while they are changing. However, if you use sysctls
to fetch this data instead, you should be able to get a stable snapshot of the
system state without getting stuck in a possible loop. I believe for libkvm
to use sysctl instead of /dev/kmem you have to pass a NULL for the kernel and
"/dev/null" for the core image.
In our code, we're invoking kvm_openfiles as you suggest:
kd = kvm_openfiles (NULL, _PATH_DEVNULL, NULL, O_RDONLY, errbuf)
It sounds like this issue might be the one fixed in r272566: if the
KERN_PROC_ALL sysctl is read with an insufficiently large buffer, an
sbuf error return value could bubble up and be treated as ERESTART,
resulting in a loop.
This can be confirmed with something like
dtrace -n 'syscall:::entry /pid == $target/{@[probefunc] = count();} tick-3s
{exit(0);}' -p <pid of looping proc>
If the output consists solely of __sysctl, this bug is likely the
culprit.
Unfortunately, I accidentally killed fstat this morning before I could do any
further debug.
I ran truss -p on it yesterday and it was spinning solely on __sysctl.
I'll try compiling with debug symbols in case it happens again. I haven't been able to reproduce the
problem in a reasonable time frame so it could be days or weeks before we see it happen again.
-Nick
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