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

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Date:	Mon, 3 Aug 2015 11:44:00 +0000
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	lkml <linux-kernel@...r.kernel.org>,
	"arc-linux-dev@...opsys.com" <arc-linux-dev@...opsys.com>
Subject: Re: [PATCH 3/6] ARC: LLOCK/SCOND based spin_lock

On Monday 03 August 2015 04:59 PM, Peter Zijlstra wrote:
> 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."

OK ! Will change this for v2.

> 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?

Sure !

>> 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 ...

My initial experiment with removing this caused weird jumps in hackbench numbers
and i'v enot had the chance to investigate it at all - but hopefully after this
series I can go back to it. So for now I'd like to keep it as is...

>> +
>> +	__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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