linux-kernel - Re: [PATCH] [PATCH] Gaurantee spinlocks implicit barrier for !PREEMPT

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Message-ID: <CA+55aFyvD2NtQatdaFZt+Q6ZPD0USofhgZHEock8rOx0ihSLhQ@mail.gmail.com>
Date:	Sat, 6 Apr 2013 11:01:23 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	Mark Salter <msalter@...hat.com>,
	Aurelien Jacquiot <a-jacquiot@...com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Christian Ruppert <christian.ruppert@...lis.com>,
	Pierrick Hascoet <pierrick.hascoet@...lis.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <srostedt@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	linux-c6x-dev@...ux-c6x.org
Subject: Re: [PATCH] [PATCH] Gaurantee spinlocks implicit barrier for !PREEMPT_COUNT

Looking around, it looks like c6x has the same bug.

Some other architectures (tile) have such subtle implementations
(where is __insn_mtspr() defined?) that I have a hard time judging.
And maybe I missed something, but the rest seem ok.

                Linus

On Sat, Apr 6, 2013 at 9:13 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> This is all *COMPLETELY* wrong.
>
> Neither the normal preempt macros, nor the plain spinlocks, should
> protect anything at all against interrupts.
>
> The real protection should come from the  spin_lock_irqsave() in
> lock_timer_base(), not from spinlocks, and not from preemption.
>
> It sounds like ARC is completely buggered, and hasn't made the irq
> disable be a compiler barrier. That's an ARC bug, and it's a big one,
> and can affect a lot more than just the timers.
>
> So the real fix is to add a "memory" clobber to
> arch_local_irq_save/restore() and friends, so that the compiler
> doesn't get to cache memory state from the irq-enabled region into the
> irq-disabled one.
>
> Fix ARC, don't try to blame generic code. You should have asked
> yourself why only ARC saw this bug, when the code apparently works
> fine for everybody else!
>
>                    Linus
>
> On Sat, Apr 6, 2013 at 6:34 AM, Vineet Gupta <Vineet.Gupta1@...opsys.com> wrote:
>>> On 04/05/2013 10:06 AM, Vineet Gupta wrote:
>>> Hi Thomas,
>>>
>>> Given that we are closing on 3.9 release, and that one/more of these patches fix a
>>> real issue for us - can you please consider my earlier patch to fix
>>> timer_pending() only for 3.9 [http://www.spinics.net/lists/kernel/msg1508224.html]
>>> This will be a localized / low risk change for this late in cycle.
>>>
>>> For 3.10 - assuming preempt_* change is blessed, we can revert this one and add
>>> that fuller/better fix.
>>>
>>> What do you think ?
>>>
>>> Thx,
>>> -Vineet
>>>
>>
>> Ping ! Sorry for pestering, but one of the fixes is needed before 3.9 goes out.
>>
>> Simple localized fix: http://www.spinics.net/lists/kernel/msg1508224.html
>> Better but risky: http://www.spinics.net/lists/kernel/msg1510885.html
>>
>> Thx,
>> -Vineet
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