linux-kernel - Re: [PATCH v3 4/8] lockdep: Add a kernel parameter, crossrelease

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Date:   Tue, 24 Oct 2017 12:08:58 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Byungchul Park <byungchul.park@....com>
Cc:     peterz@...radead.org, axboe@...nel.dk, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, tj@...nel.org,
        johannes.berg@...el.com, oleg@...hat.com, amir73il@...il.com,
        david@...morbit.com, darrick.wong@...cle.com,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-block@...r.kernel.org, hch@...radead.org, idryomov@...il.com,
        kernel-team@....com
Subject: Re: [PATCH v3 4/8] lockdep: Add a kernel parameter,
 crossrelease_fullstack


* Byungchul Park <byungchul.park@....com> wrote:

> Make whether to allow recording full stack, in cross-release feature,
> switchable at boot time via a kernel parameter, 'crossrelease_fullstack'.
> In case of a splat with no stack trace, one could just reboot and set
> the kernel parameter to get the full data without having to recompile
> the kernel.
> 
> Change CONFIG_CROSSRELEASE_STACK_TRACE default from N to Y, and
> introduce the new kernel parameter.
> 
> Signed-off-by: Byungchul Park <byungchul.park@....com>
> ---
>  Documentation/admin-guide/kernel-parameters.txt |  3 +++
>  kernel/locking/lockdep.c                        | 18 ++++++++++++++++--
>  lib/Kconfig.debug                               |  5 +++--
>  3 files changed, 22 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index ead7f40..4107b01 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -709,6 +709,9 @@
>  			It will be ignored when crashkernel=X,high is not used
>  			or memory reserved is below 4G.
>  
> +	crossrelease_fullstack
> +			[KNL] Allow to record full stack trace in cross-release
> +
>  	cryptomgr.notests
>                          [KNL] Disable crypto self-tests
>  
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 5c2ddf2..feba887 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -76,6 +76,15 @@
>  #define lock_stat 0
>  #endif
>  
> +static int crossrelease_fullstack;
> +static int __init allow_crossrelease_fullstack(char *str)
> +{
> +	crossrelease_fullstack = 1;
> +	return 0;
> +}
> +
> +early_param("crossrelease_fullstack", allow_crossrelease_fullstack);
> +
>  /*
>   * lockdep_lock: protects the lockdep graph, the hashes and the
>   *               class/list/hash allocators.
> @@ -4864,8 +4873,13 @@ static void add_xhlock(struct held_lock *hlock)
>  	xhlock->trace.max_entries = MAX_XHLOCK_TRACE_ENTRIES;
>  	xhlock->trace.entries = xhlock->trace_entries;
>  #ifdef CONFIG_CROSSRELEASE_STACK_TRACE
> -	xhlock->trace.skip = 3;
> -	save_stack_trace(&xhlock->trace);
> +	if (crossrelease_fullstack) {
> +		xhlock->trace.skip = 3;
> +		save_stack_trace(&xhlock->trace);
> +	} else {
> +		xhlock->trace.nr_entries = 1;
> +		xhlock->trace.entries[0] = hlock->acquire_ip;
> +	}
>  #else
>  	xhlock->trace.nr_entries = 1;
>  	xhlock->trace.entries[0] = hlock->acquire_ip;
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index fe8fceb..132536d 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -1228,7 +1228,7 @@ config LOCKDEP_COMPLETIONS
>  config CROSSRELEASE_STACK_TRACE
>  	bool "Record more than one entity of stack trace in crossrelease"
>  	depends on LOCKDEP_CROSSRELEASE
> -	default n
> +	default y
>  	help
>  	 The lockdep "cross-release" feature needs to record stack traces
>  	 (of calling functions) for all acquisitions, for eventual later
> @@ -1238,7 +1238,8 @@ config CROSSRELEASE_STACK_TRACE
>  	 full analysis. This option turns on the saving of the full stack
>  	 trace entries.
>  
> -	 If unsure, say N.
> +	 To make the feature actually on, set "crossrelease_fullstack"
> +	 kernel parameter, too.
>  
>  config DEBUG_LOCKDEP
>  	bool "Lock dependency engine debugging"

This is really unnecessarily complex.

The proper logic is to introduce the crossrelease_fullstack boot parameter, and to 
also have a Kconfig option that enables it: 

	CONFIG_BOOTPARAM_LOCKDEP_CROSSRELEASE_FULLSTACK=y

No #ifdefs please - just an "if ()" branch dependent on the current value of 
crossrelease_fullstack.

Thanks,

	Ingo