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Subject: Re: [PATCH v2 02/43] powerpc/traps: Declare unrecoverable_exception()
 as __noreturn
To:     Nicholas Piggin <npiggin@gmail.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Paul Mackerras <paulus@samba.org>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
References: <cover.1615291471.git.christophe.leroy@csgroup.eu>
 <f097a1071254e8f6875588f8fb9771467824a569.1615291471.git.christophe.leroy@csgroup.eu>
 <1615339022.cb2m6h66vl.astroid@bobo.none>
From:   Christophe Leroy <christophe.leroy@csgroup.eu>
Message-ID: <9e96b084-1914-f82e-5577-183bb7082d18@csgroup.eu>
Date:   Fri, 12 Mar 2021 09:40:53 +0100
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Le 10/03/2021 à 02:22, Nicholas Piggin a écrit :
> Excerpts from Christophe Leroy's message of March 9, 2021 10:09 pm:
>> unrecoverable_exception() is never expected to return, most callers
>> have an infiniteloop in case it returns.
>>
>> Ensure it really never returns by terminating it with a BUG(), and
>> declare it __no_return.
>>
>> It always GCC to really simplify functions calling it. In the exemple
>> below, it avoids the stack frame in the likely fast path and avoids
>> code duplication for the exit.
>>
>> With this patch:
> 
> [snip]
> 
> Nice.
> 
>> diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
>> index a44a30b0688c..d5c9d9ddd186 100644
>> --- a/arch/powerpc/kernel/traps.c
>> +++ b/arch/powerpc/kernel/traps.c
>> @@ -2170,11 +2170,15 @@ DEFINE_INTERRUPT_HANDLER(SPEFloatingPointRoundException)
>>    * in the MSR is 0.  This indicates that SRR0/1 are live, and that
>>    * we therefore lost state by taking this exception.
>>    */
>> -void unrecoverable_exception(struct pt_regs *regs)
>> +void __noreturn unrecoverable_exception(struct pt_regs *regs)
>>   {
>>   	pr_emerg("Unrecoverable exception %lx at %lx (msr=%lx)\n",
>>   		 regs->trap, regs->nip, regs->msr);
>>   	die("Unrecoverable exception", regs, SIGABRT);
>> +	/* die() should not return */
>> +	WARN(true, "die() unexpectedly returned");
>> +	for (;;)
>> +		;
>>   }
> 
> I don't think the WARN should be added because that will cause another
> interrupt after something is already badly wrong, so this might just
> make it harder to debug.
> 
> For example if die() is falling through for some reason, we warn and
> cause a program check here, and that might also be unrecoverable so it
> might come through here and fall through again and warn again, etc.
> 
> Putting the infinite loop is good enough I think (and better than there
> was previously).

Ok, dropped the WARN()

> 
> Otherwise
> 
> Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
> 
> Thanks,
> Nick
>