linux-kernel - Re: [PATCH] X86: reboot-notify additions

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Message-ID: <20080619110214.GJ15228@elte.hu>
Date:	Thu, 19 Jun 2008 13:02:14 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Cliff Wickman <cpw@....com>
Cc:	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	the arch/x86 maintainers <x86@...nel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] X86: reboot-notify additions


* Cliff Wickman <cpw@....com> wrote:

> From: Cliff Wickman <cpw@....com>
> 
> X86 reboot-notify additions.
> 
> This patch adds scans of the "reboot_notifier_list" callback chain in
> a three other places where the kernel is being stopped and/or restarted.
> 
> Adds calls to blocking_notifier_call_chain() in:
>    crash_kexec(), emergency_restart(), sys_kexec_load()
> 
> In the crash_kexec() and emergency_restart() cases it is indicated to the
> called-back function that the system is not in a sane state, so that
> it can avoid taking a lock or some such potentially blocking action.
> 
> These callbacks are important to a partition system. The stopped kernel needs
> to inform other partitions of their need to disconnect (stop sharing memory).
> 
> Diffed against 2.6.26-rc6
> 
> Signed-off-by: Cliff Wickman <cpw@....com>
> ---
>  include/linux/notifier.h |    4 ++++
>  kernel/kexec.c           |    5 +++++
>  kernel/sys.c             |    1 +
>  3 files changed, 10 insertions(+)
> 
> Index: linux/include/linux/notifier.h
> ===================================================================
> --- linux.orig/include/linux/notifier.h
> +++ linux/include/linux/notifier.h
> @@ -202,6 +202,10 @@ static inline int notifier_to_errno(int 
>  #define SYS_RESTART	SYS_DOWN
>  #define SYS_HALT	0x0002	/* Notify of system halt */
>  #define SYS_POWER_OFF	0x0003	/* Notify of system power off */
> +#define SYS_INSANE	0x0004	/* Notify of system error/panic/oops */
> +/* For the SYS_INSANE case, no locks should be taken by the called-back
> + * function.  The kernel is ready for an immediate reboot.
> + */
>  
>  #define NETLINK_URELEASE	0x0001	/* Unicast netlink socket released */
>  
> Index: linux/kernel/kexec.c
> ===================================================================
> --- linux.orig/kernel/kexec.c
> +++ linux/kernel/kexec.c
> @@ -1001,6 +1001,9 @@ asmlinkage long sys_kexec_load(unsigned 
>  		if (result)
>  			goto out;
>  	}
> +
> +	blocking_notifier_call_chain(&reboot_notifier_list, SYS_RESTART, NULL);
> +
>  	/* Install the new kernel, and  Uninstall the old */
>  	image = xchg(dest_image, image);
>  
> @@ -1068,6 +1071,8 @@ void crash_kexec(struct pt_regs *regs)
>  	if (!locked) {
>  		if (kexec_crash_image) {
>  			struct pt_regs fixed_regs;
> +			blocking_notifier_call_chain(&reboot_notifier_list,
> +				SYS_INSANE, NULL);
>  			crash_setup_regs(&fixed_regs, regs);
>  			crash_save_vmcoreinfo();
>  			machine_crash_shutdown(&fixed_regs);
> Index: linux/kernel/sys.c
> ===================================================================
> --- linux.orig/kernel/sys.c
> +++ linux/kernel/sys.c
> @@ -270,6 +270,7 @@ out_unlock:
>   */
>  void emergency_restart(void)
>  {
> +	blocking_notifier_call_chain(&reboot_notifier_list, SYS_INSANE, NULL);
>  	machine_emergency_restart();
>  }
>  EXPORT_SYMBOL_GPL(emergency_restart);

i dont think this is a good idea. reboot_notifier_list is a blocking 
notifier, i.e. it comes with a notifier->rwsem read-write mutex that is 
taken when blocking_notifier_call_chain() is executed.

i.e. this patch puts a sleeping mutex operation (a down_read()) into a 
highly critical code path of the kernel. This will decrease the 
reliability of the kernel.

what exactly are you trying to achieve?

	Ingo
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