Re: [Linux-v4.6-rc1] ext4: WARNING: CPU: 2 PID: 2692 at kernel/locking/lockdep.c:2017 __lock_acquire+0x180e/0x2260

Boqun Feng
2016-03-30T10:00:47Z

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Hi Peter,

On Wed, Mar 30, 2016 at 11:36:59AM +0200, Peter Zijlstra wrote:
> On Tue, Mar 29, 2016 at 10:47:02AM +0200, Ingo Molnar wrote:
>
> > > You are right; this is lockdep running into a hash collision; which is a new
> > > DEBUG_LOCKDEP test. See 9e4e7554e755 ("locking/lockdep: Detect chain_key
> > > collisions").
> >
> > I've Cc:-ed Alfredo Alvarez Fernandez who added that test.
>
> OK, so while the code in check_no_collision() seems sensible, it relies
> on borken bits.
>
> The whole chain_hlocks and /proc/lockdep_chains stuff appears to have
> been buggered from the start.
>
> The below patch should fix this.
>
> Furthermore, our hash function has definite room for improvement.
>
> ---
> include/linux/lockdep.h | 8 +++++---
> kernel/locking/lockdep.c | 30 ++++++++++++++++++++++++------
> kernel/locking/lockdep_proc.c | 2 ++
> 3 files changed, 31 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
> index d026b190c530..2568c120513b 100644
> --- a/include/linux/lockdep.h
> +++ b/include/linux/lockdep.h
> @@ -196,9 +196,11 @@ struct lock_list {
> * We record lock dependency chains, so that we can cache them:
> */
> struct lock_chain {
> - u8 irq_context;
> - u8 depth;
> - u16 base;
> + /* see BUILD_BUG_ON()s in lookup_chain_cache() */
> + unsigned int irq_context : 2,
> + depth : 6,
> + base : 24;
> + /* 4 byte hole */
> struct hlist_node entry;
> u64 chain_key;
> };
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 53ab2f85d77e..91a4b7780afb 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c

[...]

> @@ -2860,11 +2882,6 @@ static int separate_irq_context(struct task_struct *curr,
> {
> unsigned int depth = curr->lockdep_depth;
>
> - /*
> - * Keep track of points where we cross into an interrupt context:
> - */
> - hlock->irq_context = 2*(curr->hardirq_context ? 1 : 0) +
> - curr->softirq_context;
> if (depth) {
> struct held_lock *prev_hlock;
>
> @@ -3164,6 +3181,7 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
> hlock->acquire_ip = ip;
> hlock->instance = lock;
> hlock->nest_lock = nest_lock;
> + hlock->irq_context = 2*(!!curr->hardirq_context) + !!curr->softirq_context;
> hlock->trylock = trylock;
> hlock->read = read;
> hlock->check = check;

This is just for cleaning up, right? However ->hardirq_context and
->softirq_context only defined when CONFIG_TRACE_IRQFLAGS=y.

So we should use macro like current_hardirq_context() here? Or
considering the two helpers introduced in my RFC:

http://lkml.kernel.org/g/1455602265-16490-2-git-send-email-boqun.feng@gmail.com

if you don't think that overkills ;-)

Regards,
Boqun

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