On Wed, 2 Sep 2020 18:03:23 +0200
pet...@infradead.org wrote:
>
> During the LPC RCU BoF Paul asked how come the "USED" <- "IN-NMI"
> detector doesn't trip over rcu_read_lock()'s lockdep annotation.
>
> Looking into this I found a very embarrasing typo in
> verify_lock_unused():
>
> - if (!(class->usage_mask & LOCK_USED))
> + if (!(class->usage_mask & LOCKF_USED))
>
> fixing that will indeed cause rcu_read_lock() to insta-splat :/
>
> The above typo means that instead of testing for: 0x100 (1 <<
> LOCK_USED), we test for 8 (LOCK_USED), which corresponds to (1 <<
> LOCK_ENABLED_HARDIRQ).
>
> So instead of testing for _any_ used lock, it will only match any lock
> used with interrupts enabled.
>
> The rcu_read_lock() annotation uses .check=0, which means it will not
> set any of the interrupt bits and will thus never match.
>
> In order to properly fix the situation and allow rcu_read_lock() to
> correctly work, split LOCK_USED into LOCK_USED and LOCK_USED_READ and by
> having .read users set USED_READ and test USED, pure read-recursive
> locks are permitted.
Confirmed this makes lockdep warnings on kretprobes spinlocks correctly.
Tested-by: Masami Hiramatsu <mhira...@kernel.org>
Note, if we backport this, we need to backport kretprobe lockless series too.
Thank you,
>
> Fixes: f6f48e180404 ("lockdep: Teach lockdep about "USED" <- "IN-NMI"
> inversions")
> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
> ---
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index cccf4bc759c6..454355c033d2 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -4324,13 +4324,18 @@ static int separate_irq_context(struct task_struct
> *curr,
> static int mark_lock(struct task_struct *curr, struct held_lock *this,
> enum lock_usage_bit new_bit)
> {
> - unsigned int new_mask = 1 << new_bit, ret = 1;
> + unsigned int old_mask, new_mask, ret = 1;
>
> if (new_bit >= LOCK_USAGE_STATES) {
> DEBUG_LOCKS_WARN_ON(1);
> return 0;
> }
>
> + if (new_bit == LOCK_USED && this->read)
> + new_bit = LOCK_USED_READ;
> +
> + new_mask = 1 << new_bit;
> +
> /*
> * If already set then do not dirty the cacheline,
> * nor do any checks:
> @@ -4343,13 +4348,22 @@ static int mark_lock(struct task_struct *curr, struct
> held_lock *this,
> /*
> * Make sure we didn't race:
> */
> - if (unlikely(hlock_class(this)->usage_mask & new_mask)) {
> - graph_unlock();
> - return 1;
> - }
> + if (unlikely(hlock_class(this)->usage_mask & new_mask))
> + goto unlock;
>
> + old_mask = hlock_class(this)->usage_mask;
> hlock_class(this)->usage_mask |= new_mask;
>
> + /*
> + * Save one usage_traces[] entry and map both LOCK_USED and
> + * LOCK_USED_READ onto the same entry.
> + */
> + if (new_bit == LOCK_USED || new_bit == LOCK_USED_READ) {
> + if (old_mask & (LOCKF_USED | LOCKF_USED_READ))
> + goto unlock;
> + new_bit = LOCK_USED;
> + }
> +
> if (!(hlock_class(this)->usage_traces[new_bit] = save_trace()))
> return 0;
>
> @@ -4363,6 +4377,7 @@ static int mark_lock(struct task_struct *curr, struct
> held_lock *this,
> return 0;
> }
>
> +unlock:
> graph_unlock();
>
> /*
> @@ -5297,12 +5312,20 @@ static void verify_lock_unused(struct lockdep_map
> *lock, struct held_lock *hlock
> {
> #ifdef CONFIG_PROVE_LOCKING
> struct lock_class *class = look_up_lock_class(lock, subclass);
> + unsigned long mask = LOCKF_USED;
>
> /* if it doesn't have a class (yet), it certainly hasn't been used yet
> */
> if (!class)
> return;
>
> - if (!(class->usage_mask & LOCK_USED))
> + /*
> + * READ locks only conflict with USED, such that if we only ever use
> + * READ locks, there is no deadlock possible -- RCU.
> + */
> + if (!hlock->read)
> + mask |= LOCKF_USED_READ;
> +
> + if (!(class->usage_mask & mask))
> return;
>
> hlock->class_idx = class - lock_classes;
> diff --git a/kernel/locking/lockdep_internals.h
> b/kernel/locking/lockdep_internals.h
> index baca699b94e9..b0be1560ed17 100644
> --- a/kernel/locking/lockdep_internals.h
> +++ b/kernel/locking/lockdep_internals.h
> @@ -19,6 +19,7 @@ enum lock_usage_bit {
> #include "lockdep_states.h"
> #undef LOCKDEP_STATE
> LOCK_USED,
> + LOCK_USED_READ,
> LOCK_USAGE_STATES
> };
>
> @@ -40,6 +41,7 @@ enum {
> #include "lockdep_states.h"
> #undef LOCKDEP_STATE
> __LOCKF(USED)
> + __LOCKF(USED_READ)
> };
>
> #define LOCKDEP_STATE(__STATE) LOCKF_ENABLED_##__STATE |
--
Masami Hiramatsu <mhira...@kernel.org>
