linux-kernel - Re: [PATCH 5/6] ARCv2: spinlock/rwlock/atomics: Delayed retry of failed SCOND with exponential backoff

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Date:	Mon, 3 Aug 2015 13:41:04 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:	lkml <linux-kernel@...r.kernel.org>, arc-linux-dev@...opsys.com
Subject: Re: [PATCH 5/6] ARCv2: spinlock/rwlock/atomics: Delayed retry of
 failed SCOND with exponential backoff

On Mon, Aug 03, 2015 at 03:33:07PM +0530, Vineet Gupta wrote:
> +#define SCOND_FAIL_RETRY_VAR_DEF						\
> +	unsigned int delay = 1, tmp;						\
> +
> +#define SCOND_FAIL_RETRY_ASM							\
> +	"	bz	4f			\n"				\
> +	"   ; --- scond fail delay ---		\n"				\
> +	"	mov	%[tmp], %[delay]	\n"	/* tmp = delay */	\
> +	"2: 	brne.d	%[tmp], 0, 2b		\n"	/* while (tmp != 0) */	\
> +	"	sub	%[tmp], %[tmp], 1	\n"	/* tmp-- */		\
> +	"	asl	%[delay], %[delay], 1	\n"	/* delay *= 2 */	\
> +	"	b	1b			\n"	/* start over */	\
> +	"4: ; --- success ---			\n"				\
> +
> +#define SCOND_FAIL_RETRY_VARS							\
> +	  ,[delay] "+&r" (delay),[tmp] "=&r"	(tmp)				\
> +
> +#define ATOMIC_OP(op, c_op, asm_op)					\
> +static inline void atomic_##op(int i, atomic_t *v)			\
> +{									\
> +	unsigned int val, delay = 1, tmp;				\

Maybe use your SCOND_FAIL_RETRY_VAR_DEF ?

> +									\
> +	__asm__ __volatile__(						\
> +	"1:	llock   %[val], [%[ctr]]		\n"		\
> +	"	" #asm_op " %[val], %[val], %[i]	\n"		\
> +	"	scond   %[val], [%[ctr]]		\n"		\
> +	"						\n"		\
> +	SCOND_FAIL_RETRY_ASM						\
> +									\
> +	: [val]	"=&r"	(val) /* Early clobber to prevent reg reuse */	\
> +	  SCOND_FAIL_RETRY_VARS						\
> +	: [ctr]	"r"	(&v->counter), /* Not "m": llock only supports reg direct addr mode */	\
> +	  [i]	"ir"	(i)						\
> +	: "cc");							\
> +}									\
> +
> +#define ATOMIC_OP_RETURN(op, c_op, asm_op)				\
> +static inline int atomic_##op##_return(int i, atomic_t *v)		\
> +{									\
> +	unsigned int val, delay = 1, tmp;				\

Idem.

> +									\
> +	/*								\
> +	 * 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"		\
> +	"	" #asm_op " %[val], %[val], %[i]	\n"		\
> +	"	scond   %[val], [%[ctr]]		\n"		\
> +	"						\n"		\
> +	SCOND_FAIL_RETRY_ASM						\
> +									\
> +	: [val]	"=&r"	(val)						\
> +	  SCOND_FAIL_RETRY_VARS						\
> +	: [ctr]	"r"	(&v->counter),					\
> +	  [i]	"ir"	(i)						\
> +	: "cc");							\
> +									\
> +	smp_mb();							\
> +									\
> +	return val;							\
> +}

> +#define SCOND_FAIL_RETRY_VAR_DEF						\
> +	unsigned int delay, tmp;						\
> +
> +#define SCOND_FAIL_RETRY_ASM							\
> +	"   ; --- scond fail delay ---		\n"				\
> +	"	mov	%[tmp], %[delay]	\n"	/* tmp = delay */	\
> +	"2: 	brne.d	%[tmp], 0, 2b		\n"	/* while (tmp != 0) */	\
> +	"	sub	%[tmp], %[tmp], 1	\n"	/* tmp-- */		\
> +	"	asl	%[delay], %[delay], 1	\n"	/* delay *= 2 */	\
> +	"	b	1b			\n"	/* start over */	\
> +	"					\n"				\
> +	"4: ; --- done ---			\n"				\
> +
> +#define SCOND_FAIL_RETRY_VARS							\
> +	  ,[delay] "=&r" (delay), [tmp] "=&r"	(tmp)				\

This is looking remarkably similar to the previous ones, why not a
shared header?

> +static inline void arch_spin_lock(arch_spinlock_t *lock)
> +{
> +	unsigned int val;
> +	SCOND_FAIL_RETRY_VAR_DEF;
> +
> +	smp_mb();
> +
> +	__asm__ __volatile__(
> +	"0:	mov	%[delay], 1		\n"
> +	"1:	llock	%[val], [%[slock]]	\n"
> +	"	breq	%[val], %[LOCKED], 1b	\n"	/* spin while LOCKED */
> +	"	scond	%[LOCKED], [%[slock]]	\n"	/* acquire */
> +	"	bz	4f			\n"	/* done */
> +	"					\n"
> +	SCOND_FAIL_RETRY_ASM

But,... in the case that macro is empty, the label 4 does not actually
exist. I see no real reason for this to be different from the previous
incarnation either.

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