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SubjectRe: BUG: using __this_cpu_read() in preemptible code in trace_hardirqs_on
On Wed, 21 Oct 2020 15:17:33 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> > And I'm also guessing that we can call this with interrupts enabled (based
> > on the comment).
> >
> > And we have this:
> >
> > local_irq_enable()
> > trace_hardirqs_on()
> > lockdep_hardirqs_on()
> > __this_cpu_read()
>
> Moo, two threads..
>
> 20201019183355.GS2611@hirez.programming.kicks-ass.net

But this one's much older ;-)

>
> ---
>
> On Tue, Oct 20, 2020 at 12:55:46AM +0800, kernel test robot wrote:
> > [ 92.898145] BUG: using __this_cpu_read() in preemptible [00000000] code: trinity-c6/526
>
> > [ 92.903305] Call Trace:
> > [ 92.905182] __this_cpu_preempt_check+0xf/0x11
> > [ 92.905968] lockdep_hardirqs_on_prepare+0x2c/0x18f
> > [ 92.906853] trace_hardirqs_on+0x49/0x53
> > [ 92.907578] __bad_area_nosemaphore+0x3a/0x134
>
> Hurph, that's a spurious local_irq_enable(). I suppose this'll fix it.
>
> ---
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 3e99dfef8408..9f818145ef7d 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -4057,9 +4057,6 @@ void lockdep_hardirqs_on_prepare(unsigned long ip)
> if (unlikely(in_nmi()))
> return;
>
> - if (unlikely(__this_cpu_read(lockdep_recursion)))
> - return;
> -
> if (unlikely(lockdep_hardirqs_enabled())) {

Hmm, would moving the recursion check below the check of the
lockdep_hardirqs_enable() cause a large skew in the spurious enable stats?
May not be an issue, but something we should check to make sure that
there's not a path that constantly hits this.

-- Steve

> /*
> * Neither irq nor preemption are disabled here
> @@ -4070,6 +4067,9 @@ void lockdep_hardirqs_on_prepare(unsigned long ip)
> return;
> }
>
> + if (unlikely(__this_cpu_read(lockdep_recursion)))
> + return;
> +

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