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

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Message-ID: <20160330095006.GC11035@twins.programming.kicks-ass.net>
Date:	Wed, 30 Mar 2016 11:50:06 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	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

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.

Note that unless we had more than 65536 chain_hlocks consumed the patch
would not make a difference.

> Furthermore, our hash function has definite room for improvement.

And no matter how good we make it, a u64 hash is bound to collide at
some point (or of any size really).

Also, we could make them non-fatal, returning true from
lookup_chain_cache() is always correct (_very_ expensive, but correct),
so in case of doubt we could just return true.