linux-kernel - Re: [PATCH v2] powerpc: Fix irq_soft_mask_set() and irq_soft_mask

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Date:   Fri, 2 Sep 2022 10:57:27 -0500
From:   Peter Bergner <bergner@...ux.ibm.com>
To:     Segher Boessenkool <segher@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Zhouyi Zhou <zhouzhouyi@...il.com>
Subject: Re: [PATCH v2] powerpc: Fix irq_soft_mask_set() and
 irq_soft_mask_return() with sanitizer

On 8/31/22 5:45 PM, Segher Boessenkool wrote:
> On Tue, Aug 30, 2022 at 09:10:02AM +0000, Christophe Leroy wrote:
>> Le 30/08/2022 à 11:01, Nicholas Piggin a écrit :
>>> On Tue Aug 30, 2022 at 3:24 PM AEST, Christophe Leroy wrote:
>>>>> This is still slightly concerning to me. Is there any guarantee that the
>>>>> compiler would not use a different sequence for the address here?
>>>>>
>>>>> Maybe explicit r13 is required.
>>>>>
>>>>
>>>> local_paca is defined as:
>>>>
>>>> 	register struct paca_struct *local_paca asm("r13");
> 
> And this is in global scope, making it a global register variable.
> 
>>>> Why would the compiler use another register ?
>>>
>>> Hopefully it doesn't. Is it guaranteed that it won't?
> 
> Yes, this is guaranteed.

Agree with Segher here.  That said, there was a gcc bug a looooong time
ago where gcc copied r13 into a temporary register and used it from there.
That's ok (correctness wise, but not ideal) from user land standpoint,
but we took a context switch after the reg copy and it was restarted on
a different cpu, so differnt local_paca and r13 value.  We went boom
because the copy wasn't pointing to the correct local_paca anymore.
So it is very important the compiler always use r13 when accessing
the local_paca.

Peter