generic_stop_cpus: prevent parallel execution

[John Baldwin]

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.
> 
> 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.

-- 
John Baldwin