linux-kernel - Re: [Linux-v4.6-rc1] ext4: WARNING: CPU: 2 PID: 2692 at kernel/locking/lockdep.c:2017 __lock

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Date:	Wed, 30 Mar 2016 17:59:54 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Alfredo Alvarez Fernandez <alfredoalvarezfernandez@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Sedat Dilek <sedat.dilek@...il.com>,
	Theodore Ts'o <tytso@....edu>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Linux-v4.6-rc1] ext4: WARNING: CPU: 2 PID: 2692 at
 kernel/locking/lockdep.c:2017 __lock_acquire+0x180e/0x2260

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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