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: <20160330131521.GE3430@twins.programming.kicks-ass.net>
Date:	Wed, 30 Mar 2016 15:15:21 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Sedat Dilek <sedat.dilek@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Alfredo Alvarez Fernandez <alfredoalvarezfernandez@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 02:46:36PM +0200, Sedat Dilek wrote:
>  dependency chain hlocks:             49608 [max: 327680]

OK, so that is still below the u16 limit, so you're seeing an actual
hash collision and my patch will not cure that.

A different hash function _might_ help, but eventually this is an
unfixable problem. Our input space is (2^13)^48 = 2^(13*48) = 2^624 =
ff'n huge, reducing that to 2^64 is bound to generate a collision at some
point.

[ technically the 48 held_lock spots are not fully independent, so
  (2^13)^48 is slightly overestimating it, but the numbers are big
  enough for this to not matter much. ]