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Subject: Re: [PATCH] md: dm-writeback: add __noreturn to BUG-ging function
To:     Christian Borntraeger <borntraeger@de.ibm.com>,
        linux-kernel@vger.kernel.org
Cc:     Mikulas Patocka <mpatocka@redhat.com>,
        Alasdair Kergon <agk@redhat.com>,
        Mike Snitzer <snitzer@redhat.com>, dm-devel@redhat.com,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>, linux-s390@vger.kernel.org
References: <20201113225228.20563-1-rdunlap@infradead.org>
 <344abf76-9405-58ba-2dc4-27cab88c974d@de.ibm.com>
From:   Randy Dunlap <rdunlap@infradead.org>
Message-ID: <c29eeb5d-0683-49eb-f729-38b14fac7745@infradead.org>
Date:   Mon, 16 Nov 2020 15:00:01 -0800
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On 11/15/20 11:30 PM, Christian Borntraeger wrote:
> 
> 
> On 13.11.20 23:52, Randy Dunlap wrote:
>> Building on arch/s390/ flags this as an error, so add the
>> __noreturn attribute modifier to prevent the build error.
>>
>> cc1: some warnings being treated as errors
>> ../drivers/md/dm-writecache.c: In function 'persistent_memory_claim':
>> ../drivers/md/dm-writecache.c:323:1: error: no return statement in function returning non-void [-Werror=return-type]
> 
> ok with me, but I am asking why
> 
> the unreachable macro is not good enough. For x86 it obviously is.
> 
> form arch/s390/include/asm/bug.h
> #define BUG() do {                                      \
>         __EMIT_BUG(0);                                  \
>         unreachable();                                  \
> } while (0)
> 

Hi Christian,

Good question.
I don't see any guidance about when to use one or the other etc.

I see __noreturn being used 109 times and unreachable();
being used 33 times, but only now that I look at them.
That had nothing to do with why I used __noreturn in the patch.

> 
>>
>> Fixes: 48debafe4f2f ("dm: add writecache target")
>> Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
>> Cc: Mikulas Patocka <mpatocka@redhat.com>
>> Cc: Alasdair Kergon <agk@redhat.com>
>> Cc: Mike Snitzer <snitzer@redhat.com>
>> Cc: dm-devel@redhat.com
>> Cc: Heiko Carstens <hca@linux.ibm.com>
>> Cc: Vasily Gorbik <gor@linux.ibm.com>
>> Cc: Christian Borntraeger <borntraeger@de.ibm.com>
>> Cc: linux-s390@vger.kernel.org
>> ---
>>  drivers/md/dm-writecache.c |    2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> --- linux-next-20201113.orig/drivers/md/dm-writecache.c
>> +++ linux-next-20201113/drivers/md/dm-writecache.c
>> @@ -317,7 +317,7 @@ err1:
>>  	return r;
>>  }
>>  #else
>> -static int persistent_memory_claim(struct dm_writecache *wc)
>> +static int __noreturn persistent_memory_claim(struct dm_writecache *wc)
>>  {
>>  	BUG();
>>  }
>>

thanks.
-- 
~Randy