I was tricked into finishing the work by Andrey Gapon, who developed the patch to reliably stop other processors on panic. The patch greatly improves the chances of getting dump on panic on SMP host. Several people already saw the patchset, and I remember that Andrey posted it to some lists.
The change stops other (*) processors early upon the panic. This way, no parallel manipulation of the kernel memory is performed by CPUs. In particular, the kernel memory map is static. Patch prevents the panic thread from blocking and switching out. * - in the context of the description, other means not current. Since other threads are not run anymore, lock owner cannot release a lock which is required by panic thread. Due to this, we need to fake a lock acquisition after the panic, which adds minimal overhead to the locking cost. The patch tries to not add any overhead on the fast path of the lock acquire. The check for the after-panic condition was reduced to single memory access, done only when the quick cas lock attempt failed, and braced with __unlikely compiler hint. For now, the new mode of operation is disabled by default, since some further USB changes are needed to make USB keyboard usable in that environment. With the patch, getting a dump from the machine without debugger compiled in is much more realistic. Please comment, I will commit the change in 2 weeks unless strong reasons not to are given. http://people.freebsd.org/~kib/misc/stop_cpus_on_panic.1.patch
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