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    SubjectRe: [git pull] scheduler fixes
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    On Sat, Jan 31, 2009 at 20:54, Ingo Molnar <mingo@elte.hu> wrote:
    >
    > * 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:

    Is there actually a reason for a separate softirq stack on i386-4K, or
    any other architecture?


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