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    SubjectRe: [git pull] scheduler fixes

    * 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


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