RE: [PATCH 18/30] notifier: Show function names on notifier routines if DEBUG_NOTIFIERS is set

["Michael Kelley (LINUX)" <mikelley@microsoft.com>]

From: Guilherme G. Piccoli <gpiccoli@igalia.com> Sent: Wednesday, April 27, 2022 3:49 PM
> 
> Currently we have a debug infrastructure in the notifiers file, but
> it's very simple/limited. This patch extends it by:
> 
> (a) Showing all registered/unregistered notifiers' callback names;
> 
> (b) Adding a dynamic debug tuning to allow showing called notifiers'
> function names. Notice that this should be guarded as a tunable since
> it can flood the kernel log buffer.
> 
> Cc: Arjan van de Ven <arjan@linux.intel.com>
> Cc: Cong Wang <xiyou.wangcong@gmail.com>
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Cc: Valentin Schneider <valentin.schneider@arm.com>
> Cc: Xiaoming Ni <nixiaoming@huawei.com>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
> ---
> 
> We have some design decisions that worth discussing here:
> 
> (a) First of call, using C99 helps a lot to write clear and concise code, but

s/call/all/

> due to commit 4d94f910e79a ("Kbuild: use -Wdeclaration-after-statement") we
> have a warning if mixing variable declarations with code. For this patch though,
> doing that makes the code way clear, so decision was to add the debug code
> inside brackets whenever this warning pops up. We can change that, but that'll
> cause more ifdefs in the same function.
> 
> (b) In the symbol lookup helper function, we modify the parameter passed but
> even more, we return it as well! This is unusual and seems unnecessary, but was
> the strategy taken to allow embedding such function in the pr_debug() call.
> 
> Not doing that would likely requiring 3 symbol_name variables to avoid
> concurrency (registering notifier A while calling notifier B) - we rely in
> local variables as a serialization mechanism.
> 
> We're open for suggestions in case this design is not appropriate;
> thanks in advance!
> 
>  kernel/notifier.c | 48 +++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 46 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/notifier.c b/kernel/notifier.c
> index ba005ebf4730..21032ebcde57 100644
> --- a/kernel/notifier.c
> +++ b/kernel/notifier.c
> @@ -7,6 +7,22 @@
>  #include <linux/vmalloc.h>
>  #include <linux/reboot.h>
> 
> +#ifdef CONFIG_DEBUG_NOTIFIERS
> +#include <linux/kallsyms.h>
> +
> +/*
> + *	Helper to get symbol names in case DEBUG_NOTIFIERS is set.
> + *	Return the modified parameter is a strategy used to achieve
> + *	the pr_debug() functionality - with this, function is only
> + *	executed if the dynamic debug tuning is effectively set.
> + */
> +static inline char *notifier_name(struct notifier_block *nb, char *sym_name)
> +{
> +	lookup_symbol_name((unsigned long)(nb->notifier_call), sym_name);
> +	return sym_name;
> +}
> +#endif
> +
>  /*
>   *	Notifier list for kernel code which wants to be called
>   *	at shutdown. This is used to stop any idling DMA operations
> @@ -34,20 +50,41 @@ static int notifier_chain_register(struct notifier_block **nl,
>  	}
>  	n->next = *nl;
>  	rcu_assign_pointer(*nl, n);
> +
> +#ifdef CONFIG_DEBUG_NOTIFIERS
> +	{
> +		char sym_name[KSYM_NAME_LEN];
> +
> +		pr_info("notifiers: registered %s()\n",
> +			notifier_name(n, sym_name));
> +	}
> +#endif
>  	return 0;
>  }
> 
>  static int notifier_chain_unregister(struct notifier_block **nl,
>  		struct notifier_block *n)
>  {
> +	int ret = -ENOENT;
> +
>  	while ((*nl) != NULL) {
>  		if ((*nl) == n) {
>  			rcu_assign_pointer(*nl, n->next);
> -			return 0;
> +			ret = 0;
> +			break;
>  		}
>  		nl = &((*nl)->next);
>  	}
> -	return -ENOENT;
> +
> +#ifdef CONFIG_DEBUG_NOTIFIERS
> +	if (!ret) {
> +		char sym_name[KSYM_NAME_LEN];
> +
> +		pr_info("notifiers: unregistered %s()\n",
> +			notifier_name(n, sym_name));
> +	}
> +#endif
> +	return ret;
>  }
> 
>  /**
> @@ -80,6 +117,13 @@ static int notifier_call_chain(struct notifier_block **nl,
>  			nb = next_nb;
>  			continue;
>  		}
> +
> +		{
> +			char sym_name[KSYM_NAME_LEN];
> +
> +			pr_debug("notifiers: calling %s()\n",
> +				 notifier_name(nb, sym_name));
> +		}
>  #endif
>  		ret = nb->notifier_call(nb, val, v);
> 
> --
> 2.36.0