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

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Message-ID: <20180830142920.GO24082@hirez.programming.kicks-ass.net>
Date:   Thu, 30 Aug 2018 16:29:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     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 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.