linux-kernel - Re: [RFC][PATCH 03/31] locking,arc: Implement atomic_fetch

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Date:	Mon, 25 Apr 2016 04:26:54 +0000
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Peter Zijlstra <peterz@...radead.org>
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Subject: Re: [RFC][PATCH 03/31] locking,arc: Implement
 atomic_fetch_{add,sub,and,andnot,or,xor}()

On Friday 22 April 2016 07:46 PM, Peter Zijlstra wrote:
> On Fri, Apr 22, 2016 at 10:50:41AM +0000, Vineet Gupta wrote:
>
>>> > > +#define ATOMIC_FETCH_OP(op, c_op, asm_op)				\
>>> > > +static inline int atomic_fetch_##op(int i, atomic_t *v)			\
>>> > > +{									\
>>> > > +	unsigned int val, result;			                \
>>> > > +	SCOND_FAIL_RETRY_VAR_DEF                                        \
>>> > > +									\
>>> > > +	/*								\
>>> > > +	 * Explicit full memory barrier needed before/after as		\
>>> > > +	 * LLOCK/SCOND thmeselves don't provide any such semantics	\
>>> > > +	 */								\
>>> > > +	smp_mb();							\
>>> > > +									\
>>> > > +	__asm__ __volatile__(						\
>>> > > +	"1:	llock   %[val], [%[ctr]]		\n"		\
>>> > > +	"	mov %[result], %[val]			\n"		\
>> > 
>> > Calling it result could be a bit confusing, this is meant to be the "orig" value.
>> > So it indeed "result" of the API, but for atomic operation it is pristine value.
>> > 
>> > Also we can optimize away that MOV - given there are plenty of regs, so
>> > 
>>> > > +	"	" #asm_op " %[val], %[val], %[i]	\n"		\
>>> > > +	"	scond   %[val], [%[ctr]]		\n"		\
>> > 
>> > Instead have
>> > 
>> > +	"	" #asm_op " %[result], %[val], %[i]	\n"		\
>> > +	"	scond   %[result], [%[ctr]]		\n"		\
>> > 
>> > 
> Indeed, how about something like so?
>
> ---
> Subject: locking,arc: Implement atomic_fetch_{add,sub,and,andnot,or,xor}()
> From: Peter Zijlstra <peterz@...radead.org>
> Date: Mon Apr 18 01:16:09 CEST 2016
>
> Implement FETCH-OP atomic primitives, these are very similar to the
> existing OP-RETURN primitives we already have, except they return the
> value of the atomic variable _before_ modification.
>
> This is especially useful for irreversible operations -- such as
> bitops (because it becomes impossible to reconstruct the state prior
> to modification).
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Acked-by: Vineet Gupta <vgupta@...opsys.com>