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* for_each_cpu()  is buggy for UP kernel?
@ 2018-05-09  6:24 Dexuan Cui
  2018-05-09 23:20 ` Andrew Morton
                   ` (2 more replies)
  0 siblings, 3 replies; 10+ messages in thread
From: Dexuan Cui @ 2018-05-09  6:24 UTC (permalink / raw)
  To: Ingo Molnar, Alexey Dobriyan, Andrew Morton, Peter Zijlstra,
	Thomas Gleixner, Greg Kroah-Hartman, Rakib Mullick
  Cc: ''

In include/linux/cpumask.h, for_each_cpu is defined like this for UP kernel (CONFIG_NR_CPUS=1):

#define for_each_cpu(cpu, mask)                 \
        for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask)

Here 'mask' is ignored, but what if 'mask' contains 0 CPU? -- in this case, the for loop should not
run at all, but with the current code, we run the loop once with cpu==0.

I think I'm seeing a bug in my UP kernel that is caused by the buggy for_each_cpu():

in kernel/time/tick-broadcast.c: tick_handle_oneshot_broadcast(), tick_broadcast_oneshot_mask
contains 0 CPU, but due to the buggy for_each_cpu(), the variable 'next_event' is changed from
its default value KTIME_MAX to "next_event = td->evtdev->next_event"; as a result,
tick_handle_oneshot_broadcast () -> tick_broadcast_set_event() -> clockevents_program_event()
-> pit_next_event() is programming the PIT timer by accident, causing an interrupt storm of PIT
interrupts in some way: I'm seeing that the kernel is receiving ~8000 PIT interrupts per second for
1~5 minutes when the UP kernel boots, and it looks the kernel hangs, but in 1~5 minutes, finally
somehow the kernel can recover and boot up fine. But, occasionally, the kernel just hangs there
forever, receiving ~8000 PIT timers per second. 

With the below change in kernel/time/tick-broadcast.c, the interrupt storm will go away:

+#undef for_each_cpu
+#define for_each_cpu(cpu, mask)                        \
+       for ((cpu) = 0; (((cpu) < 1) && ((mask)[0].bits[0] & 1)); (cpu)++, (void)mask)

Should we fix the for_each_cpu() in include/linux/cpumask.h for UP?

-- Dexuan

^ permalink raw reply	[flat|nested] 10+ messages in thread

end of thread, other threads:[~2018-05-15 20:48 UTC | newest]

Thread overview: 10+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2018-05-09  6:24 for_each_cpu() is buggy for UP kernel? Dexuan Cui
2018-05-09 23:20 ` Andrew Morton
2018-05-13 13:35   ` Thomas Gleixner
2018-05-13 18:21 ` Linus Torvalds
2018-05-14  7:28   ` Dmitry Vyukov
2018-05-15  3:02   ` Dexuan Cui
2018-05-15 17:21     ` Linus Torvalds
2018-05-15 20:10       ` Dexuan Cui
2018-05-15 19:52 ` [PATCH] tick/broadcast: Use for_each_cpu() specially on UP kernels Dexuan Cui
2018-05-15 20:48   ` [tip:timers/urgent] " tip-bot for Dexuan Cui

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