linux-kernel - Re: [PATCH v2 1/2] spinlock: New spinlock_refcount.h for lockless update of refcount

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Date:	Wed, 26 Jun 2013 22:17:13 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Waiman Long <Waiman.Long@...com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Jeff Layton <jlayton@...hat.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Ingo Molnar <mingo@...hat.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH v2 1/2] spinlock: New spinlock_refcount.h for lockless
 update of refcount

> + * The combined data structure is 8-byte aligned. So proper placement of this
> + * structure in the larger embedding data structure is needed to ensure that
> + * there is no hole in it.

On i386 u64 is only 4 bytes aligned. So you need to explicitely align
it to 8 bytes. Otherwise you risk the two members crossing a cache line, which
would be really expensive with atomics.

> +	/*
> +	 * Code doesn't work if raw spinlock is larger than 4 bytes
> +	 * or is empty.
> +	 */
> +	BUG_ON((sizeof(arch_spinlock_t) > 4) || (sizeof(arch_spinlock_t) == 0));

BUILD_BUG_ON 

> +
> +	spin_unlock_wait(plock);	/* Wait until lock is released */
> +	old.__lock_count = ACCESS_ONCE(*plockcnt);
> +	get_lock = ((threshold >= 0) && (old.count == threshold));
> +	if (likely(!get_lock && spin_can_lock(&old.lock))) {

What is that for? Why can't you do the CMPXCHG unconditially ?

If it's really needed, it is most likely a race?

The duplicated code should be likely an inline.

> +/*
> + * The presence of either one of the CONFIG_DEBUG_SPINLOCK or
> + * CONFIG_DEBUG_LOCK_ALLOC configuration parameter will force the
> + * spinlock_t structure to be 8-byte aligned.
> + *
> + * To support the spinlock/reference count combo data type for 64-bit SMP
> + * environment with spinlock debugging turned on, the reference count has
> + * to be integrated into the spinlock_t data structure in this special case.
> + * The spinlock_t data type will be 8 bytes larger if CONFIG_GENERIC_LOCKBREAK
> + * is also defined.

I would rather just disable the optimization when these CONFIGs are set

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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