linux-kernel - Re: [PATCH 1/4] ARC: add barriers to futex code

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Date:	Thu, 6 Aug 2015 15:48:26 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Michel Lespinasse <walken@...gle.com>,
	arc-linux-dev@...opsys.com, lkml <linux-kernel@...r.kernel.org>,
	David Hildenbrand <dahi@...ux.vnet.ibm.com>,
	Will Deacon <will.deacon@....com>, ralf@...ux-mips.org
Subject: Re: [PATCH 1/4] ARC: add barriers to futex code

On Thu, Aug 06, 2015 at 06:05:20PM +0530, Vineet Gupta wrote:
> The atomic ops on futex need to provide the full barrier just like
> regular atomics in kernel.
> 
> Also remove pagefault_enable/disable in futex_atomic_cmpxchg_inatomic()
> as core code already does that

Urgh, and of course tglx just left for holidays :-)

> +++ b/arch/arc/include/asm/futex.h
> @@ -20,6 +20,7 @@
>  
>  #define __futex_atomic_op(insn, ret, oldval, uaddr, oparg)\
>  							\
> +	smp_mb();					\
>  	__asm__ __volatile__(				\
>  	"1:	llock	%1, [%2]		\n"	\
>  		insn				"\n"	\
> @@ -40,12 +41,14 @@
>  							\
>  	: "=&r" (ret), "=&r" (oldval)			\
>  	: "r" (uaddr), "r" (oparg), "ir" (-EFAULT)	\
> -	: "cc", "memory")
> +	: "cc", "memory");				\
> +	smp_mb();					\
>  


So:

 - alhpa: only has the first smp_mb(), suggesting RELEASE
 - arm: only has the first smp_mb(), suggesting RELEASE
 - arm64: has store-release + smp_mb(), suggesting full barriers
 - MIPS: has LLSC_MB after, suggesting ACQUIRE
 - powerpc: lwsync before, sync after, full barrier

x86 is of course boring and fully ordered

Looking at the usage site of futex_atomic_op_inuser(), that's in
futex_wake_op() which might suggest RELEASE is indeed sufficient.

Which leaves me puzzled on MIPS, but what do I know.

At the very least this patch isn't wrong, fully ordered is sufficient.
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