linux-kernel - Re: [PATCH 14/30] panic: Properly identify the panic event to the notifiers' callbacks

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Date:   Tue, 17 May 2022 15:11:10 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     "Guilherme G. Piccoli" <gpiccoli@...lia.com>
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Subject: Re: [PATCH 14/30] panic: Properly identify the panic event to the
 notifiers' callbacks

On Tue 2022-05-10 13:16:54, Guilherme G. Piccoli wrote:
> On 10/05/2022 12:16, Petr Mladek wrote:
> > [...]
> > Hmm, this looks like a hack. PANIC_UNUSED will never be used.
> > All notifiers will be always called with PANIC_NOTIFIER.
> > 
> > The @val parameter is normally used when the same notifier_list
> > is used in different situations.
> > 
> > But you are going to use it when the same notifier is used
> > in more lists. This is normally distinguished by the @nh
> > (atomic_notifier_head) parameter.
> > 
> > IMHO, it is a bad idea. First, it would confuse people because
> > it does not follow the original design of the parameters.
> > Second, the related code must be touched anyway when
> > the notifier is moved into another list so it does not
> > help much.
> > 
> > Or do I miss anything, please?
> > 
> > Best Regards,
> > Petr
> 
> Hi Petr, thanks for the review.
> 
> I'm not strong attached to this patch, so we could drop it and refactor
> the code of next patches to use the @nh as identification - but
> personally, I feel this parameter could be used to identify the list
> that called such function, in other words, what is the event that
> triggered the callback. Some notifiers are even declared with this
> parameter called "ev", like the event that triggers the notifier.
> 
> 
> You mentioned 2 cases:
> 
> (a) Same notifier_list used in different situations;
> 
> (b) Same *notifier callback* used in different lists;
> 
> Mine is case (b), right? Can you show me an example of case (a)?

There are many examples of case (a):

   + module_notify_list:
	MODULE_STATE_LIVE, 	/* Normal state. */
	MODULE_STATE_COMING,	/* Full formed, running module_init. */
	MODULE_STATE_GOING,	/* Going away. */
	MODULE_STATE_UNFORMED,	/* Still setting it up. */


   + netdev_chain:

	NETDEV_UP	= 1,	/* For now you can't veto a device up/down */
	NETDEV_DOWN,
	NETDEV_REBOOT,		/* Tell a protocol stack a network interface
				   detected a hardware crash and restarted
				   - we can use this eg to kick tcp sessions
				   once done */
	NETDEV_CHANGE,		/* Notify device state change */
	NETDEV_REGISTER,
	NETDEV_UNREGISTER,
	NETDEV_CHANGEMTU,	/* notify after mtu change happened */
	NETDEV_CHANGEADDR,	/* notify after the address change */
	NETDEV_PRE_CHANGEADDR,	/* notify before the address change */
	NETDEV_GOING_DOWN,
	...

    + vt_notifier_list:

	#define VT_ALLOCATE		0x0001 /* Console got allocated */
	#define VT_DEALLOCATE		0x0002 /* Console will be deallocated */
	#define VT_WRITE		0x0003 /* A char got output */
	#define VT_UPDATE		0x0004 /* A bigger update occurred */
	#define VT_PREWRITE		0x0005 /* A char is about to be written to the console */

    + die_chain:

	DIE_OOPS = 1,
	DIE_INT3,
	DIE_DEBUG,
	DIE_PANIC,
	DIE_NMI,
	DIE_DIE,
	DIE_KERNELDEBUG,
	...

These all call the same list/chain in different situations.
The situation is distinguished by @val.


> You can see in the following patches (or grep the kernel) that people are using
> this identification parameter to determine which kind of OOPS trigger
> the callback to condition the execution of the function to specific
> cases.

Could you please show me some existing code for case (b)?
I am not able to find any except in your patches.

Anyway, the solution in 16th patch is bad, definitely.
hv_die_panic_notify_crash() uses "val" to disinguish
both:

     + "panic_notifier_list" vs "die_chain"
     + die_val when callen via "die_chain"

The API around "die_chain" API is not aware of enum panic_notifier_val
and the API using "panic_notifier_list" is not aware of enum die_val.
As I said, it is mixing apples and oranges and it is error prone.

Best Regards,
Petr