[Bug 243711] dtrace immediately panics the system if compiled without SMP

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=243711

--- Comment #2 from alex_y_xu at yahoo.ca ---
(In reply to Mark Johnston from comment #1)

> Which revision are you running?

I used
https://download.freebsd.org/ftp/snapshots/VM-IMAGES/13.0-CURRENT/amd64/Latest/FreeBSD-13.0-CURRENT-amd64.qcow2.xz
(date 2020-Jan-23 09:41) and
https://download.freebsd.org/ftp/snapshots/amd64/13.0-CURRENT/src.txz (date
2020-Jan-23 08:56). My understanding is that these are kept up-to-date, but
probably your source copy is newer.

> I can't quite see the issue from looking at the DTrace code.

I also tried some debugging, but I am not familiar with the FreeBSD kernel. I
suspect that the backtrace is not accurate. With FreeBSD 12.1-STABLE, I
acquired a backtrace that had some dtrace state=non-zero, then magically become
arg=0x0 when it reached dtrace_xcall or thereabouts. Maybe something strange is
going on with the registers during the trap and panic though. I didn't get
around to trying DDB though.

> why are you using a non-SMP kernel?

I want to use a non-SMP kernel because I am on a single-CPU VM and I assume
that no-SMP kernels are more efficient. On Linux, spinlocks in
interrupt-disabled context can be compiled out of the kernel in no-SMP mode. If
it's really poorly tested though, there's no particular reason I *can't* use an
SMP kernel.

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