RE: problem in kzalloc or rt_spin_lock_fastlock?

["Sydir, Jerry" <jerry.sydir@intel.com>]

>-----Original Message-----
>From: Manikandan Ramachandran [mailto:crmanik@gmail.com]
>Sent: Friday, September 10, 2010 9:51 AM
>To: Sydir, Jerry
>Cc: linux-rt-users@vger.kernel.org
>Subject: Re: problem in kzalloc or rt_spin_lock_fastlock?
>
>i did come across this issue, in different context. I worked around
>this by not using kzalloc, instead by having static allocation.

I was able to cobble together a version of oprofile in which I have replaced the calls to kzalloc/kfree with static allocations. Its not an elegant solution, but it seems to work.

I am still hoping that somebody can answer my question about whether this is a real bug (a bug where things are really going wrong) or whether might_sleep is being called unnecessarily. What would be the symptom if the call to kzalloc (or kfree) did sleep? Would the system hang? Does that fact that it makes it through the call without any apparent symptoms besides the kernel oops mean that things are OK?

Jerry

>
>I guess this is one of the RT related changes to make atomic calls
>preemptible.
>
>HTH,
>Mani
>
>
>On Thu, Sep 9, 2010 at 2:28 PM, Sydir, Jerry <jerry.sydir@intel.com> wrote:
>> Hello all,
>>
>> I had posted a question a few weeks ago concerning problems with perf and
>oprofile when running on the 2.6.33.7-rt29 kernel. I received help on how
>to fix the problem with perf, but no replies about oprofile. Having looked
>into the oprofile problem more carefully, I have a more specific question
>that I'm hoping somebody can answer.
>>
>>
>> Below is the stack trace that I received when starting the oprofile
>daemon. (It's the same trace that I included in my previous post). Filling
>in some of the macros and inlined functions the call sequence is as
>follows: Ppro_setup_cntrs calls kzalloc with the GFP_ATOMIC flag set.
>According to kmalloc documentation this means that the call should not
>sleep. Kmalloc calls kmem_cache_alloc_notrace, calls kmem_cache_alloc,
>calls __cache_alloc, calls _slab_irq_disable, calls get_cpu_val_locked (a
>macro in which a call to rt_spin_lock is generated). Rt_spin_lock calls
>rt_spin_lock_fastlock which has the following code:
>>
>>   /* Temporary HACK! */
>> 706         if (likely(!current->in_printk))
>> 707                 might_sleep();
>> 708         else if (in_atomic() || irqs_disabled())
>> 709                 /* don't grab locks for printk in atomic */
>> 710                 return;
>>
>> Might_sleep is called and the kernel notes this as an error. I'm guessing
>that since this call did not originate in printk, might_sleep is getting
>called.
>>
>> I have very little knowledge of the inner workings of the kernel, so I
>don't know whether the error is with one of the functions in the call
>sequence, or with the check on line 706 which results in an unnecessary
>call to might_sleep(). I have found that although the kernel reports an
>oops, the oprofile daemon and the system seem to operate correctly. Can
>someone please confirm whether there is a real error here or whether the
>problem is that might_sleep is being called unnecessarily in this case? Can
>I assume that all is well with oprofile and ignore the kernel oops?
>>
>> Thanks in advance for your help.
>> Jerry Sydir
>>
>>
>> oprofile: using NMI interrupt.
>> BUG: sleeping function called from invalid context at
>kernel/rtmutex.c:707
>> pcnt: 2 0 in_atomic(): 1, irqs_disabled(): 1, pid: 4020, name: oprofiled
>> Pid: 4020, comm: oprofiled Not tainted 2.6.33.7-rt29 #3
>> Call Trace:
>> [<c104e8c4>] ? rt_spin_lock_fastlock+0x26/0x58
>> [<c108ff56>] ? _slab_irq_disable+0x22/0x42
>> [<c109108a>] ? __kmalloc+0x7d/0xf0
>> [<e0071529>] ? ppro_setup_ctrs+0x29/0x1b8 [oprofile]
>> [<e0071529>] ? ppro_setup_ctrs+0x29/0x1b8 [oprofile]
>> [<e0070e8d>] ? nmi_cpu_setup+0x87/0xcc [oprofile]
>> [<e0070e06>] ? nmi_cpu_setup+0x0/0xcc [oprofile]
>> [<c102fa7b>] ? on_each_cpu+0x25/0x50
>> [<e0070dd9>] ? nmi_setup+0x11d/0x14a [oprofile]
>> [<e006f133>] ? oprofile_setup+0x2d/0x86 [oprofile]
>> [<e006fede>] ? event_buffer_open+0x42/0x60 [oprofile]
>> [<c109363d>] ? __dentry_open+0x1a4/0x29a
>> [<c109b09b>] ? generic_permission+0xc/0x7e
>> [<c10937c1>] ? nameidata_to_filp+0x27/0x38
>> [<e006fe9c>] ? event_buffer_open+0x0/0x60 [oprofile]
>> [<c109d214>] ? do_filp_open+0x439/0x843
>> [<c10808f6>] ? __do_fault+0x2bf/0x2ef
>> [<c1090091>] ? slab_irq_enable+0x45/0x79
>> [<c10933a2>] ? do_sys_open+0x4c/0xe4
>> [<c109347e>] ? sys_open+0x1e/0x23
>> [<c1002750>] ? sysenter_do_call+0x12/0x26
>> --
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>
>
>
>--
>Thanks,
>Manik
>
>Think twice about a tree before you take a printout
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