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From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: Alfredo Alvarez Fernandez <alfredoalvarezfernandez@gmail.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Sedat Dilek <sedat.dilek@gmail.com>,
	Theodore Ts'o <tytso@mit.edu>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [Linux-v4.6-rc1] ext4: WARNING: CPU: 2 PID: 2692 at kernel/locking/lockdep.c:2017 __lock_acquire+0x180e/0x2260
Date: Wed, 30 Mar 2016 11:50:06 +0200	[thread overview]
Message-ID: <20160330095006.GC11035@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20160330093659.GS3408@twins.programming.kicks-ass.net>

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.



  parent reply	other threads:[~2016-03-30  9:50 UTC|newest]

Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-03-27  8:15 [Linux-v4.6-rc1] ext4: WARNING: CPU: 2 PID: 2692 at kernel/locking/lockdep.c:2017 __lock_acquire+0x180e/0x2260 Sedat Dilek
2016-03-27  8:57 ` Sedat Dilek
2016-03-27 12:03   ` Linus Torvalds
2016-03-27 13:32     ` Boqun Feng
2016-03-27 18:23     ` Theodore Ts'o
2016-03-27 19:40       ` Sedat Dilek
     [not found]         ` <CA+55aFwoKRpgq8OCTxUaP+8gOg-mnN3nbruYgiK32a=C5U4TkQ@mail.gmail.com>
2016-03-27 20:24           ` Sedat Dilek
2016-03-27 20:48     ` Peter Zijlstra
2016-03-27 20:59       ` Sedat Dilek
2016-03-27 21:48         ` Sedat Dilek
2016-03-28  1:05         ` Boqun Feng
2016-03-28  6:33           ` Peter Zijlstra
2016-03-29  8:47       ` Ingo Molnar
2016-03-30  9:20         ` Sedat Dilek
2016-03-30  9:36           ` Sedat Dilek
2016-03-30  9:36         ` Peter Zijlstra
2016-03-30  9:49           ` Sedat Dilek
2016-03-30 10:33             ` Sedat Dilek
2016-03-30 12:43             ` Peter Zijlstra
2016-03-30 12:46               ` Sedat Dilek
2016-03-30 13:15                 ` Peter Zijlstra
2016-03-30  9:50           ` Peter Zijlstra [this message]
2016-03-30  9:59           ` Boqun Feng
2016-03-30 10:36             ` Peter Zijlstra
2016-03-30 11:07               ` Sedat Dilek
2016-03-31 15:42             ` Peter Zijlstra
2016-03-31 15:52               ` Boqun Feng
2016-04-02  6:26               ` Sedat Dilek
2016-03-30 14:06           ` Peter Zijlstra
2016-03-30 15:21             ` Sedat Dilek
2016-03-30 17:03               ` [PATCH] lockdep: print chain_key collision information Alfredo Alvarez Fernandez
2016-03-30 17:19                 ` Peter Zijlstra
2016-04-01  6:36                 ` [tip:core/urgent] locking/lockdep: Print " tip-bot for Alfredo Alvarez Fernandez
2016-05-10  9:09                   ` Peter Zijlstra
2016-04-04 15:31             ` [Linux-v4.6-rc1] ext4: WARNING: CPU: 2 PID: 2692 at kernel/locking/lockdep.c:2017 __lock_acquire+0x180e/0x2260 Sedat Dilek
2016-04-04 16:02               ` Peter Zijlstra
2016-05-09 11:37                 ` Sedat Dilek
2016-06-03 15:15             ` Sedat Dilek
2016-04-23 12:54           ` [tip:locking/urgent] lockdep: Fix lock_chain::base size tip-bot for Peter Zijlstra

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