[BUG] perf: hard lockup when using perf-sched

[Li Zefan <lizf@cn.fujitsu.com>]

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An idea came to me that running perf-sched with a fork "bomb" might
reveal some bug.

So I wrote a program (attached), and did the following test:

# gcc fork.c
# ./a.out &
# perf sched record

After a few seconds, the machine froze, without printing any message.

I've tried the test both on v2.6.34-rc1 and v2.6.34-rc1-tip+.

I'll send you any other information you need.

[-- Attachment #2: fork.c --]
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#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>

int main(void)
{
	int i;
	int pid;

	while (1) {
		for (i = 0; i < 400; i++) {
			pid = fork();
			if (pid == 0) {
				return 0;
			} else if (pid < 0) {
				fprintf(stderr, "faild to fork!\n");
				continue;
			}
		}

		for (i = 0; i < 400; i++)
			if (wait(NULL) < 0)
				break;
	}

	return 0;
}