linux-kernel - Re: [PATCH v2] powerpc/32: Fix unpaired stwcx. on interrupt exit

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Message-ID: <73b75b17-adde-44f0-b252-5a7ea2f9d138@csgroup.eu>
Date: Fri, 12 Sep 2025 15:34:50 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Segher Boessenkool <segher@...nel.crashing.org>
Cc: Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin
 <npiggin@...il.com>, Madhavan Srinivasan <maddy@...ux.ibm.com>,
 linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2] powerpc/32: Fix unpaired stwcx. on interrupt exit

Hi Segher,

Le 12/09/2025 à 15:24, Segher Boessenkool a écrit :
> Hi!
> 
> On Fri, Sep 12, 2025 at 10:37:34AM +0200, Christophe Leroy wrote:
>>   BEGIN_FTR_SECTION
>> +	lwarx   r0,0,r1
>> +END_FTR_SECTION_IFSET(CPU_FTR_NEED_PAIRED_STWCX)
>>   	stwcx.	r0,0,r1		/* to clear the reservation */
>> -FTR_SECTION_ELSE
>> -	lwarx	r0,0,r1
>> -ALT_FTR_SECTION_END_IFCLR(CPU_FTR_STCX_CHECKS_ADDRESS)
> 
> Hrm.  So this is for V'ger (mpc7450)?  What kind of issues does that
> old thing have with unpaired larx/stcx., some performance impact?

No idea, The original commit (from 2007) only says "some processors can 
have issues if this stwcx to address A occurs while the reservation is 
already held to a different address B".

> 
> The extra "dummy" larx has serious performance impact itself, on most
> implementations (also on all newer stuff).

To be honnest I don't know. I just discovered I made a mistake when I 
implemented C interrupt exit and this patch is aiming at restoring the 
previous behaviour.

If you think this is pointless then no problem, I can instead just 
remove the impossible case and that's it.

What is odd to begin with is that we have two features that seems to 
address the same problem but in a slightly different way
- CPU_FTR_NEED_PAIRED_STWCX for PPC32
- CPU_FTR_STCX_CHECKS_ADDRESS for PPC64

What do you recommend ?

Christophe