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SubjectRe: [PATCH 13/14] lockdep: Prepare for NMI IRQ state tracking
On Fri, 29 May 2020 23:27:41 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> There is no reason not to always, accurately, track IRQ state.
>
> This change also makes IRQ state tracking ignore lockdep_off().
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> kernel/locking/lockdep.c | 33 ++++++++++++++++++++++++++++++---
> 1 file changed, 30 insertions(+), 3 deletions(-)
>
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -3646,7 +3646,13 @@ static void __trace_hardirqs_on_caller(v
> */
> void lockdep_hardirqs_on_prepare(unsigned long ip)
> {
> - if (unlikely(!debug_locks || current->lockdep_recursion))

Why remove the check for debug_locks? Isn't that there to disable
everything at once to prevent more warnings to be printed?

Also, isn't there other ways that we could have recursion besides NMIs?
Say we do a printk inside here, or call something that may also enable
interrupts? I thought the recursion check was also to prevent lockdep
infrastructure calling something that lockdep monitors being a problem?

Or am I missing something?

-- Steve


> + /*
> + * NMIs do not (and cannot) track lock dependencies, nothing to do.
> + */
> + if (in_nmi())
> + return;
> +
> + if (DEBUG_LOCKS_WARN_ON(current->lockdep_recursion & LOCKDEP_RECURSION_MASK))
> return;
>
> if (unlikely(current->hardirqs_enabled)) {
> @@ -3692,7 +3698,24 @@ void noinstr lockdep_hardirqs_on(unsigne
> {
> struct task_struct *curr = current;
>
> - if (unlikely(!debug_locks || curr->lockdep_recursion))
> + /*
> + * NMIs can happen in the middle of local_irq_{en,dis}able() where the
> + * tracking state and hardware state are out of sync.
> + *
> + * NMIs must save lockdep_hardirqs_enabled() to restore IRQ state from,
> + * and not rely on hardware state like normal interrupts.
> + */
> + if (in_nmi()) {
> + /*
> + * Skip:
> + * - recursion check, because NMI can hit lockdep;
> + * - hardware state check, because above;
> + * - chain_key check, see lockdep_hardirqs_on_prepare().
> + */
> + goto skip_checks;
> + }
> +
> + if (DEBUG_LOCKS_WARN_ON(curr->lockdep_recursion & LOCKDEP_RECURSION_MASK))
> return;
>
> if (curr->hardirqs_enabled) {
> @@ -3720,6 +3743,7 @@ void noinstr lockdep_hardirqs_on(unsigne
> DEBUG_LOCKS_WARN_ON(current->hardirq_chain_key !=
> current->curr_chain_key);
>
> +skip_checks:
> /* we'll do an OFF -> ON transition: */
> curr->hardirqs_enabled = 1;
> curr->hardirq_enable_ip = ip;
> @@ -3735,7 +3759,10 @@ void noinstr lockdep_hardirqs_off(unsign
> {
> struct task_struct *curr = current;
>
> - if (unlikely(!debug_locks || curr->lockdep_recursion))
> + /*
> + * NMIs can happen in lockdep.
> + */
> + if (!in_nmi() && DEBUG_LOCKS_WARN_ON(curr->lockdep_recursion & LOCKDEP_RECURSION_MASK))
> return;
>
> /*
>

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