From: Darren Hart <dvhltc@us.ibm.com>
To: Michael Neuling <mikey@neuling.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>,
Gautham R Shenoy <ego@in.ibm.com>,
Josh Triplett <josh@joshtriplett.org>,
Steven Rostedt <rostedt@goodmis.org>,
linuxppc-dev@ozlabs.org, Will Schmidt <willschm@us.ibm.com>,
Paul Mackerras <paulus@samba.org>,
Ankita Garg <ankita@in.ibm.com>, Darren Hart <darren@dvhart.com>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH][RFC] preempt_count corruption across H_CEDE call with CONFIG_PREEMPT on pseries
Date: Thu, 02 Sep 2010 17:08:39 -0700 [thread overview]
Message-ID: <4C803C87.2090209@us.ibm.com> (raw)
In-Reply-To: <30721.1283468693@neuling.org>
On 09/02/2010 04:04 PM, Michael Neuling wrote:
> In message <1283400367.2356.69.camel@gandalf.stny.rr.com> you wrote:
>> On Thu, 2010-09-02 at 11:02 +1000, Michael Neuling wrote:
>>
>>> We need to call smp_startup_cpu on boot when we the cpus are still in
>>> FW. smp_startup_cpu does this for us on boot.
>>>
>>> I'm wondering if we just need to move the test down a bit to make sure
>>> the preempt_count is set. I've not been following this thread, but
>>> maybe this might work?
>>
>> Egad no! Setting the preempt_count to zero _is_ the bug. I think Darren
>> even said that adding the exit prevented the bug (although now he's
>> hitting a hard lockup someplace else). The original code he was using
>> did not have the condition to return for kexec. It was just a
>> coincidence that this code helped in bringing a CPU back online.
>>
>>>
>>> Untested patch below...
>>>
>>> Mikey
>>>
>>> diff --git a/arch/powerpc/platforms/pseries/smp.c b/arch/powerpc/platforms/
> pseries/smp.c
>>> index 0317cce..3afaba4 100644
>>> --- a/arch/powerpc/platforms/pseries/smp.c
>>> +++ b/arch/powerpc/platforms/pseries/smp.c
>>> @@ -104,18 +104,18 @@ static inline int __devinit smp_startup_cpu(unsigned
> int lcpu)
>>>
>>> pcpu = get_hard_smp_processor_id(lcpu);
>>>
>>> - /* Check to see if the CPU out of FW already for kexec */
>>> - if (smp_query_cpu_stopped(pcpu) == QCSS_NOT_STOPPED){
>>> - cpumask_set_cpu(lcpu, of_spin_mask);
>>> - return 1;
>>> - }
>>> -
>>> /* Fixup atomic count: it exited inside IRQ handler. */
>>> task_thread_info(paca[lcpu].__current)->preempt_count = 0;
>>
>> We DON'T want to do the above. It's nasty! This is one CPU's task
>> touching an intimate part of another CPU's task. It's equivalent of me
>> putting my hand down you wife's blouse. It's offensive, and rude.
>>
>> OK, if the CPU was never online, then you can do what you want. But what
>> we see is that this fails on CPU hotplug. You stop a CPU, and it goes
>> into this cede_processor() call. When you wake it up, suddenly the task
>> on that woken CPU has its preempt count fscked up. This was really
>> really hard to debug. We thought it was stack corruption or something.
>> But it ended up being that this code has one CPU touching the breasts of
>> another CPU. This code is a pervert!
>>
>> What the trace clearly showed, was that we take down a CPU, and in doing
>> so, the code on that CPU set the preempt count to 1, and it expected to
>> have it as 1 when it returned. But the code that kicked started this CPU
>> back to life (bring the CPU back online), set the preempt count on the
>> task of that CPU to 0, and screwed everything up.
>
> /me goes to checks where this came from...
>
> It's been in the kernel since hotplug CPU support was added to ppc64
> back in 2004, so I guess we are all at fault for letting this pervert
> get away with this stuff for so long in plain sight. :-)
>
> So I guess we should remove this but we need to audit all the different
> paths that go through here to make sure they are OK with preempt.
> Normal boot, kexec boot, hotplug with FW stop and hotplug with
> extended_cede all hit this.
>
> Mikey
CC'ing my alter ego.
--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
next prev parent reply other threads:[~2010-09-03 0:08 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-07-22 18:24 [PATCH][RFC] preempt_count corruption across H_CEDE call with CONFIG_PREEMPT on pseries Darren Hart
2010-07-22 18:36 ` Darren Hart
2010-07-22 18:38 ` Thomas Gleixner
2010-08-10 22:36 ` Darren Hart
2010-07-22 22:25 ` Benjamin Herrenschmidt
2010-07-22 23:57 ` Darren Hart
2010-07-23 4:44 ` Darren Hart
2010-07-23 5:08 ` Vaidyanathan Srinivasan
2010-07-23 5:11 ` Benjamin Herrenschmidt
2010-07-23 7:07 ` Vaidyanathan Srinivasan
2010-08-05 4:45 ` Darren Hart
2010-08-05 11:06 ` Vaidyanathan Srinivasan
2010-08-05 12:26 ` Thomas Gleixner
2010-07-23 5:09 ` Benjamin Herrenschmidt
2010-08-06 2:19 ` Darren Hart
2010-08-06 5:09 ` Vaidyanathan Srinivasan
2010-08-06 7:13 ` Darren Hart
2010-07-23 14:39 ` Will Schmidt
2010-08-04 13:44 ` Darren Hart
2010-08-19 15:58 ` Ankita Garg
2010-08-19 18:58 ` Will Schmidt
2010-08-23 22:20 ` Darren Hart
2010-08-31 7:12 ` Darren Hart
2010-09-01 5:54 ` Michael Ellerman
2010-09-01 15:10 ` Darren Hart
2010-09-01 18:47 ` Darren Hart
2010-09-01 19:59 ` Steven Rostedt
2010-09-01 20:42 ` Darren Hart
2010-09-02 1:02 ` Michael Neuling
2010-09-02 4:06 ` Steven Rostedt
2010-09-02 6:04 ` Darren Hart
2010-09-03 20:10 ` Will Schmidt
2010-09-02 23:04 ` Michael Neuling
2010-09-03 0:08 ` Darren Hart [this message]
2010-09-02 3:46 ` Michael Neuling
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