linux-kernel - LOCKDEP customizable numbers upper limit

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Date:   Thu, 13 May 2021 22:39:51 +0900
From:   "J. R. Okajima" <hooanon05g@...il.com>
To:     peterz@...radead.org, penguin-kernel@...ove.SAKURA.ne.jp
Cc:     linux-kernel@...r.kernel.org
Subject: LOCKDEP customizable numbers upper limit

Hello,

According to the commit in v5.13-rc1,
	5dc33592e9553 2021-04-05 lockdep: Allow tuning tracing capacity constants.
several lockdep numbers have their own range as 10--30.
But if we set all 30s, we got a compilation error.

kernel/locking/lockdep.c:3536:2: note: in expansion of macro 'BUILD_BUG_ON'
  BUILD_BUG_ON((1UL << 24) <= ARRAY_SIZE(chain_hlocks));

kernel/locking/lockdep.c
----------------------------------------
static u16 chain_hlocks[MAX_LOCKDEP_CHAIN_HLOCKS];
	:::
	BUILD_BUG_ON((1UL << 24) <= ARRAY_SIZE(chain_hlocks));
	:::
----------------------------------------

MAX_LOCKDEP_CHAIN_HLOCKS is defined in kernel/locking/lockdep_internal.h
as this.

#define MAX_LOCKDEP_CHAINS_BITS		CONFIG_LOCKDEP_CHAINS_BITS
#define MAX_LOCKDEP_CHAINS		(1UL << MAX_LOCKDEP_CHAINS_BITS)
#define MAX_LOCKDEP_CHAIN_HLOCKS	(MAX_LOCKDEP_CHAINS*5)

I don't know what this 'multiply by 5' means and why
ARRAY_SIZE(chain_hlocks) is limited to (1UL << 24), but setting 30 to
CONFIG_LOCKDEP_CHAINS_BITS obviously causes BUILD_BUG.
'*5' is more than 2 bits shift, so CONFIG_LOCKDEP_CHAINS_BITS has to be
less than (24-2), limited to the range 10--21.

Hmm, I tried.

CONFIG_LOCKDEP_BITS=30
CONFIG_LOCKDEP_CHAINS_BITS=21
CONFIG_LOCKDEP_STACK_TRACE_BITS=30
CONFIG_LOCKDEP_STACK_TRACE_HASH_BITS=30
CONFIG_LOCKDEP_CIRCULAR_QUEUE_BITS=30

Arg, LD failed.
ld: kernel/locking/lockdep.o: in function `lockdep_hlock_class':
lockdep.c:(.text+0x84f): relocation truncated to fit: R_X86_64_PC32 against `.bss'

I am afraid these LOCKDEP configurations need some sort of balancing.

J. R. Okajima