Re: [PATCH 08/12] x86/dumpstack: Pin the target stack in save_stack_trace_tsk()

[Peter Zijlstra]

On Fri, Sep 16, 2016 at 08:12:40AM -0700, Andy Lutomirski wrote:
> On Fri, Sep 16, 2016 at 12:47 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Thu, Sep 15, 2016 at 02:19:38PM -0500, Josh Poimboeuf wrote:

> >> My idea was to use task_rq_lock() to lock the runqueue and then check
> >> tsk->on_cpu.  I think Peter wasn't too keen on it.
> >
> > That basically allows a DoS on the scheduler, since a user can run tasks
> > on every cpu (through sys_sched_setaffinity()). Then doing while (1) cat
> > /proc/$PID/stack would saturate the rq->lock on every CPU.
> >
> > The more tasks the merrier.
> 
> Is this worse than it would be if this code used preempt_disable()
> (which I think it did until very recently)?

Much worse, since the proposed task_rq_lock() not only disables
preemption, it also disables IRQs and takes 2 locks. And hogging the
rq->lock affects other tasks their ability to schedule.