linux-kernel - Re: Patch "asm-generic/bitops/lock.h: Rewrite using atomic_fetch

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Message-ID: <C2D7FE5348E1B147BCA15975FBA23075012B090EA4@us01wembx1.internal.synopsys.com>
Date:   Thu, 30 Aug 2018 20:31:59 +0000
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>
CC:     Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Alexey.Brodkin@...opsys.com" <Alexey.Brodkin@...opsys.com>,
        "Vineet.Gupta1@...opsys.com" <Vineet.Gupta1@...opsys.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "yamada.masahiro@...ionext.com" <yamada.masahiro@...ionext.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: Patch "asm-generic/bitops/lock.h: Rewrite using atomic_fetch_"
 causes kernel crash

On 08/30/2018 07:29 AM, Peter Zijlstra wrote:
> On Thu, Aug 30, 2018 at 03:23:55PM +0100, Will Deacon wrote:
>
>> Yes, that would be worth trying. However, I also just noticed that the
>> fetch-ops (which are now used to implement test_and_set_bit_lock()) seem
>> to be missing the backwards branch in the LL/SC case. Yet another diff
>> below.
>>
>> Will
>>
>> --->8
>>
>> diff --git a/arch/arc/include/asm/atomic.h b/arch/arc/include/asm/atomic.h
>> index 4e0072730241..f06c5ed672b3 100644
>> --- a/arch/arc/include/asm/atomic.h
>> +++ b/arch/arc/include/asm/atomic.h
>> @@ -84,7 +84,7 @@ static inline int atomic_fetch_##op(int i, atomic_t *v)			\
>>  	"1:	llock   %[orig], [%[ctr]]		\n"		\
>>  	"	" #asm_op " %[val], %[orig], %[i]	\n"		\
>>  	"	scond   %[val], [%[ctr]]		\n"		\
>> -	"						\n"		\
>> +	"	bnz	1b				\n"		\
>>  	: [val]	"=&r"	(val),						\
>>  	  [orig] "=&r" (orig)						\
>>  	: [ctr]	"r"	(&v->counter),					\
> ACK!! sorry about that, no idea how I messed that up.
>
> Also, once it all works, they should look at switching to _relaxed
> atomics for LL/SC.

Indeed this is the mother of all issues, I tried and system is clearly hosed with
and works after.
What's amazing is the commit 4aef66c8ae9 which introduced it is from 2016 ;-)
Back then we had a retry branch with backoff stuff which I'd reverted for new
cores and the merge conflict somehow missed it.

@PeterZ I'll create a patch with you as author ? do I need any formal sign offs,
acks etc ?

So after this there are 2 other things to be addresses / looked at still while we
are still here.

1. After 84c6591103db __clear_bit_lock() implementation will be broken (or atleast
not consistent with what we had after), do we need to reinstate it.
2. Will's proposed change to remove the underlying issue, but the issue in #1
remains ?

-Vineet