linux-kernel - Re: [PATCH 1/23] Make register values available to panic notifiers

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Date:	Mon, 12 Apr 2010 13:45:27 +0100
From:	David Howells <dhowells@...hat.com>
To:	Russell King <rmk@....linux.org.uk>
Cc:	dhowells@...hat.com, David VomLehn <dvomlehn@...co.com>,
	linux-arch@...r.kernel.org, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/23] Make register values available to panic notifiers

Russell King <rmk@....linux.org.uk> wrote:

> Can you explain why you want this?
> 
> I'm wondering about the value of saving the registers; normally when a panic
> occurs, it's because of a well defined reason, and not because something
> went wrong in some CPU register; to put it another way, a panic() is a
> more controlled exception than a BUG() or a bad pointer dereference.

+1.

I found in FS-Cache and CacheFiles that often the things I most wanted to know
when I had something of the form:

	if (A == B)
		BUG();

was a and b, so I made the following macro:

	#define ASSERTCMP(X, OP, Y)					\
	do {								\
		if (unlikely(!((X) OP (Y)))) {				\
			printk(KERN_ERR "\n");				\
			printk(KERN_ERR "AFS: Assertion failed\n");	\
			printk(KERN_ERR "%lu " #OP " %lu is false\n",	\
			       (unsigned long)(X), (unsigned long)(Y));	\
			printk(KERN_ERR "0x%lx " #OP " 0x%lx is false\n", \
			       (unsigned long)(X), (unsigned long)(Y));	\
			BUG();						\
		}							\
	} while(0)

which I could then call like this:

	ASSERTCMP(A, ==, B);

and if the assertion failed, it prints A and B explicitly.  This is much
easier than trying to pick the values out of a register dump, especially as
the compiler may be free to clobber A or B immediately after testing them.

David
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