kmsg_dumper

Nice new tracing tools notwithstanding, kernel developers still tend to reach for printk() when trying to figure out problems. But one need not work on kernel code for very long before running into an unpleasant fact: the most interesting stuff is often printed immediately before a crash, but, for many kinds of problems, the death of the system can prevent the output of those crucial lines. It's no fun to stare at a hung system, knowing that the information needed to find the problem is probably trapped in a buffer somewhere in that system's memory.

2.6.33 will contain a new mechanism designed to help get that last bit of information out of a dying system's clutches. The developer need only set up a new "kmsg dumper" along these lines:

    #include <linux/kmsg_dump.h>

    struct kmsg_dumper {
	void (*dump)(struct kmsg_dumper *dumper, enum kmsg_dump_reason reason,
			const char *s1, unsigned long l1,
			const char *s2, unsigned long l2);
	struct list_head list;
	int registered;
    };

The dump() function will be called in the event of a crash; the two arguments s1 and s2 will have pointers to the data in the kernel's output buffer. Two pointers are needed due to the circular nature of this buffer; s1 will point to the older set of messages.

Registering and unregistering this function is a matter of calling:

    int kmsg_dump_register(struct kmsg_dumper *dumper);
    int kmsg_dump_unregister(struct kmsg_dumper *dumper);

In the 2.6.33 kernel, the "mtdoops" module has been reworked to use this new mechanism to save crash data to a flash device.

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