generic_stop_cpus: prevent parallel execution

[Jung-uk Kim]

On Monday 11 October 2010 12:11 pm, John Baldwin wrote:
> On Thursday, October 07, 2010 1:53:46 pm Andriy Gapon wrote:
> > Here is patch that applies the technique from panic() to
> > generic_stop_cpus() to prevent its parallel execution on multiple
> > CPUs:
> > http://people.freebsd.org/~avg/generic_stop_cpus.diff
> >
> > In theory this could lead to two CPUs stopping each other and
> > everyone else, and thus a total system halt.
> >
> > Also, in theory, we should have some smarter locking here,
> > because two (or more CPUs) could be stopping unrelated sets of
> > CPUs.  But in practice, it seems, this function is only used to
> > stop "all other" CPUs.  Unless I overlooked other usages, that
> > is.
> >
> > Additionally, I took this opportunity to make amd64-specific
> > suspend_cpus() function use generic_stop_cpus() instead of
> > rolling out essentially duplicate code.  I couldn't see any
> > reason no to consolidate, but perhaps I missed something.

When I wrote the code, generic_stop_cpus() didn't exist.  Now it makes 
sense, though.

> > Big thanks to Matthew and his employer for the idea and example.
>
> One note.  Use 'cpu_spinwait()' in the inner loop waiting for
> 'stopping_cpu' to change.

+1.

Jung-uk Kim