linux-kernel - Re: [PATCH 3/6] ARC: LLOCK/SCOND based spin

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Date:	Mon, 3 Aug 2015 13:29:30 +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 3/6] ARC: LLOCK/SCOND based spin_lock

On Mon, Aug 03, 2015 at 03:33:05PM +0530, Vineet Gupta wrote:
> EX causes the cache line to be in Exclusive state and if done
> concurrently by multiple cores, it keeps bouncing around.
> In LLOCK/SCOND regime, spinning only involves LLOCK which doesn't
> change the line state hence better solution.

Maybe write like:

"The EXchange instruction forces the cacheline into exclusive state
(because of the modify) and concurrent loops with it will bounce the
cacheline between the cores

Instead use LLOCK/SCOND to form the test-and-set lock implementation
since LLOCK can keep the line in shared state."

Because it wasn't clear to me what EX meant, and surely a LOAD must
change the cacheline state of it previously was in exclusive on another
core. Its just that shared is a whole lot better to spin on than
exclusive.

Also, since you're using LL/SC now, a slightly more complex lock is
trivial to implement, might I suggest you look at implementing ticket
locks next?

> Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
> Signed-off-by: Vineet Gupta <vgupta@...opsys.com>
> ---
>  arch/arc/include/asm/spinlock.h | 76 +++++++++++++++++++++++++++++++++++++----
>  1 file changed, 69 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arc/include/asm/spinlock.h b/arch/arc/include/asm/spinlock.h
> index e1651df6a93d..4f6c90a0a68a 100644
> --- a/arch/arc/include/asm/spinlock.h
> +++ b/arch/arc/include/asm/spinlock.h
> @@ -18,9 +18,68 @@
>  #define arch_spin_unlock_wait(x) \
>  	do { while (arch_spin_is_locked(x)) cpu_relax(); } while (0)
>  
> +#ifdef CONFIG_ARC_HAS_LLSC
> +
> +static inline void arch_spin_lock(arch_spinlock_t *lock)
> +{
> +	unsigned int val;
> +
> +	smp_mb();

I'm still puzzled by your need of this one ...

> +
> +	__asm__ __volatile__(
> +	"1:	llock	%[val], [%[slock]]	\n"
> +	"	breq	%[val], %[LOCKED], 1b	\n"	/* spin while LOCKED */
> +	"	scond	%[LOCKED], [%[slock]]	\n"	/* acquire */
> +	"	bnz	1b			\n"
> +	"					\n"
> +	: [val]		"=&r"	(val)
> +	: [slock]	"r"	(&(lock->slock)),
> +	  [LOCKED]	"r"	(__ARCH_SPIN_LOCK_LOCKED__)
> +	: "memory", "cc");
> +
> +	smp_mb();
> +}
> +
> +/* 1 - lock taken successfully */
> +static inline int arch_spin_trylock(arch_spinlock_t *lock)
> +{
> +	unsigned int val, got_it = 0;
> +
> +	smp_mb();

Idem.

> +
> +	__asm__ __volatile__(
> +	"1:	llock	%[val], [%[slock]]	\n"
> +	"	breq	%[val], %[LOCKED], 4f	\n"	/* already LOCKED, just bail */
> +	"	scond	%[LOCKED], [%[slock]]	\n"	/* acquire */
> +	"	bnz	1b			\n"
> +	"	mov	%[got_it], 1		\n"
> +	"4:					\n"
> +	"					\n"
> +	: [val]		"=&r"	(val),
> +	  [got_it]	"+&r"	(got_it)
> +	: [slock]	"r"	(&(lock->slock)),
> +	  [LOCKED]	"r"	(__ARCH_SPIN_LOCK_LOCKED__)
> +	: "memory", "cc");
> +
> +	smp_mb();
> +
> +	return got_it;
> +}
> +
> +static inline void arch_spin_unlock(arch_spinlock_t *lock)
> +{
> +	smp_mb();
> +
> +	lock->slock = __ARCH_SPIN_LOCK_UNLOCKED__;
> +
> +	smp_mb();

Idem.

> +}
> +
> +#else	/* !CONFIG_ARC_HAS_LLSC */
--
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