Re: [git pull] scheduler fixes

Ingo Molnar
2009-01-31T17:54:57Z

* Alexey Zaytsev <alexey.zaytsev@gmail.com> wrote:

> On Sat, Jan 31, 2009 at 20:23, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Sat, 2009-01-31 at 18:11 +0100, Peter Zijlstra wrote:
> >> > diff --git a/kernel/sched.c b/kernel/sched.c
> >> > index 52bbf1c..5686bb5 100644
> >> > --- a/kernel/sched.c
> >> > +++ b/kernel/sched.c
> >> > @@ -4440,7 +4450,7 @@ void __kprobes sub_preempt_count(int val)
> >> > /*
> >> > * Underflow?
> >> > */
> >> > - if (DEBUG_LOCKS_WARN_ON(val > preempt_count()))
> >> > + if (DEBUG_LOCKS_WARN_ON(val > preempt_count() - (!!kernel_locked())))
> >> > return;
> >> > /*
> >> > * Is the spinlock portion underflowing?
> >
> > Since the commit msg of 01e3eb8 says:
> >
> > kernel_locked() is not a valid test in IRQ context (we update the
> > BKL's ->lock_depth and the preempt count separately and non-atomicalyy),
> > so we cannot put it into the generic preempt debugging checks which
> > can run in IRQ contexts too.
> >
>
> Is the comment actually valid? From arch/x86/kernel/irq_32.c:
> do_softirq() actually does
> curctx = current_thread_info();
> irqctx = softirq_ctx[smp_processor_id()];
> irqctx->tinfo.task = curctx->task;
>
> and so does execute_on_irq_stack().
> So kernel_locked() should be valid. It corresponds to the thread
> that is being interrupted.
>
> And answering an earlier question, this happens only on i386 and only
> with 4K stacks because x86_64 dosn't have a separate softirq stack,
> so the preempt count diring the soft irq is at least IRQ_EXIT_OFFSET.
>
> (If I understood the things correctly)

Correct, on 64-bit we use the hardirq stack for softirqs too:

ENTRY(call_softirq)
CFI_STARTPROC
push %rbp
CFI_ADJUST_CFA_OFFSET 8
CFI_REL_OFFSET rbp,0
mov %rsp,%rbp
CFI_DEF_CFA_REGISTER rbp
incl PER_CPU_VAR(irq_count)
cmove PER_CPU_VAR(irq_stack_ptr),%rsp
push %rbp # backlink for old unwinder
call __do_softirq

Ingo