Re: [PATCH v4 01/27] lockdep: Teach lockdep about "USED" <- "IN-NMI" inversions

Joel Fernandes
2020-02-22T03:08:47Z

On Fri, Feb 21, 2020 at 02:34:17PM +0100, Peter Zijlstra wrote:
> nmi_enter() does lockdep_off() and hence lockdep ignores everything.
>
> And NMI context makes it impossible to do full IN-NMI tracking like we
> do IN-HARDIRQ, that could result in graph_lock recursion.

The patch makes sense to me.

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

NOTE:
Also, I was wondering if we can detect the graph_lock recursion case and
avoid doing anything bad, that way we enable more of the lockdep
functionality for NMI where possible. Not sure if the suggestion makes sense
though!

thanks,

- Joel

> However, since look_up_lock_class() is lockless, we can find the class
> of a lock that has prior use and detect IN-NMI after USED, just not
> USED after IN-NMI.
>
> NOTE: By shifting the lockdep_off() recursion count to bit-16, we can
> easily differentiate between actual recursion and off.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> kernel/locking/lockdep.c | 53 ++++++++++++++++++++++++++++++++++++++++++++---
> 1 file changed, 50 insertions(+), 3 deletions(-)
>
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -379,13 +379,13 @@ void lockdep_init_task(struct task_struc
>
> void lockdep_off(void)
> {
> - current->lockdep_recursion++;
> + current->lockdep_recursion += BIT(16);
> }
> EXPORT_SYMBOL(lockdep_off);
>
> void lockdep_on(void)
> {
> - current->lockdep_recursion--;
> + current->lockdep_recursion -= BIT(16);
> }
> EXPORT_SYMBOL(lockdep_on);
>
> @@ -575,6 +575,7 @@ static const char *usage_str[] =
> #include "lockdep_states.h"
> #undef LOCKDEP_STATE
> [LOCK_USED] = "INITIAL USE",
> + [LOCK_USAGE_STATES] = "IN-NMI",
> };
> #endif
>
> @@ -787,6 +788,7 @@ static int count_matching_names(struct l
> return count + 1;
> }
>
> +/* used from NMI context -- must be lockless */
> static inline struct lock_class *
> look_up_lock_class(const struct lockdep_map *lock, unsigned int subclass)
> {
> @@ -4463,6 +4465,34 @@ void lock_downgrade(struct lockdep_map *
> }
> EXPORT_SYMBOL_GPL(lock_downgrade);
>
> +/* NMI context !!! */
> +static void verify_lock_unused(struct lockdep_map *lock, struct held_lock *hlock, int subclass)
> +{
> + struct lock_class *class = look_up_lock_class(lock, subclass);
> +
> + /* if it doesn't have a class (yet), it certainly hasn't been used yet */
> + if (!class)
> + return;
> +
> + if (!(class->usage_mask & LOCK_USED))
> + return;
> +
> + hlock->class_idx = class - lock_classes;
> +
> + print_usage_bug(current, hlock, LOCK_USED, LOCK_USAGE_STATES);
> +}
> +
> +static bool lockdep_nmi(void)
> +{
> + if (current->lockdep_recursion & 0xFFFF)
> + return false;
> +
> + if (!in_nmi())
> + return false;
> +
> + return true;
> +}
> +
> /*
> * We are not always called with irqs disabled - do that here,
> * and also avoid lockdep recursion:
> @@ -4473,8 +4503,25 @@ void lock_acquire(struct lockdep_map *lo
> {
> unsigned long flags;
>
> - if (unlikely(current->lockdep_recursion))
> + if (unlikely(current->lockdep_recursion)) {
> + /* XXX allow trylock from NMI ?!? */
> + if (lockdep_nmi() && !trylock) {
> + struct held_lock hlock;
> +
> + hlock.acquire_ip = ip;
> + hlock.instance = lock;
> + hlock.nest_lock = nest_lock;
> + hlock.irq_context = 2; // XXX
> + hlock.trylock = trylock;
> + hlock.read = read;
> + hlock.check = check;
> + hlock.hardirqs_off = true;
> + hlock.references = 0;
> +
> + verify_lock_unused(lock, &hlock, subclass);
> + }
> return;
> + }
>
> raw_local_irq_save(flags);
> check_flags(flags);
>
>