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Date: Fri, 28 Sep 2007 14:07:54 +0200
From: Laurent Vivier <Laurent.Vivier@bull.net>
Organization: Bull S.A.S.
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To: Andrew Morton <akpm@linux-foundation.org>
Cc: Fengguang Wu <wfg@mail.ustc.edu.cn>, Andi Kleen <ak@suse.de>,
       linux-kernel@vger.kernel.org
Subject: Re: WARNING: at arch/x86_64/kernel/smp.c:397 smp_call_function_mask()
References: <20070927022220.c76a7a6e.akpm@linux-foundation.org>	<390947257.08622@ustc.edu.cn>	<46FCC0B8.7090403@bull.net>	<20070928020950.bdcad2c2.akpm@linux-foundation.org>	<46FCC6F5.2030109@bull.net> <20070928023422.77be1e71.akpm@linux-foundation.org>
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Andrew Morton wrote:
> On Fri, 28 Sep 2007 11:18:45 +0200 Laurent Vivier <Laurent.Vivier@bull.=
net> wrote:
>=20
>> Andrew Morton wrote:
>>> On Fri, 28 Sep 2007 10:52:08 +0200 Laurent Vivier <Laurent.Vivier@bul=
l.net> wrote:
>>>> Andi, is this correct ?
>>>> Andrew, should I send a patch implementing this change ?
>>> umm, I think all the smp_call_function fucntions are deadlocky if cal=
led
>>> with local interrupts disabled, regardless of whether the calling CPU=
 is in
>>> the mask.
>>>
>>> If CPU A is sending a cross-cpu call to CPU B and CPU B is sending a
>>> cross-cpu call to CPU A, and they both have local interrupts disabled=
=2E..
>> OK, so there are two errors:
>>
>> 1- one I introduce myself (without any help from anyone) where
>> smp_call_function() calls all online CPUs instead of calling all CPUs =
except itself.
>=20
> I'd be pretty surprised if one was able to write a bug like that.  You =
mean
> the CPU sends an IPI to itself and then loops around until it has servi=
ced
> that IPI?  And this works?  Wow.

In fact, it works because __smp_call_function_mask() makes :

 cpus_and(mask, mask, allbutself);

So it removes current cpu from the mask. BTW, I don't have to modify
smp_call_function(): it is correct as it is written (except from a semant=
ic
point of view... do we care about semantic ?).

> And on_each_cpu() can call the handler function twice?
>=20
> hm
>=20
>> 2- one in global_flush_tlb() which calls smp_call_function() with irqs=
 disabled.
>=20
> That would be a big bug, and surely we would already have picked it up.=

>=20
> <looks>
>=20
> argh, mainline's x86_64 smp_call_function() doesn't do the check.  We'v=
e
> had *heaps* of bugs in i386 where people were running smp_call_foo() un=
der
> local_irq_disable().  I wonder how many there are in x86_64?
>=20
>> I think I should at least correct #1 ?
>=20
> I think we should correct all bugs ;)
>=20

We don't live in a perfect world... ;-)

Moreover, I'm not able to reproduce #2 ...

Fengguang could you send me your .config and your qemu command line param=
eters ?

Laurent

PS: if semantic is important, you can apply attached patch...
--=20
------------- Laurent.Vivier@bull.net  --------------
          "Software is hard" - Donald Knuth

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