Re: [PATCH v6 01/14] spinlock: A new lockref structure for locklessupdate of refcount

[Masami Hiramatsu]

Hi,

(2013/07/09 10:09), Waiman Long wrote:> +/**
> + * lockref_put_or_lock - decrements count unless count <= 1 before decrement
> + * @lockcnt: pointer to lockref structure
> + * Return: 1 if count updated successfully or 0 if count <= 1 and lock taken
> + *
> + * The only difference between lockref_put_or_lock and lockref_put is that
> + * the former function will hold the lock on return while the latter one
> + * will free it on return.
> + */
> +static __always_inline int lockref_put_or_locked(struct lockref *lockcnt)

Here is a function name typo. _locked should be _lock.
And also, I think we should take a note here to tell this function does *not*
guarantee lockcnt->refcnt == 0 or 1 until unlocked if this returns 0.

> +{
> +	spin_lock(&lockcnt->lock);
> +	if (likely(lockcnt->refcnt > 1)) {
> +		lockcnt->refcnt--;
> +		spin_unlock(&lockcnt->lock);
> +		return 1;
> +	}
> +	return 0;
> +}

Using this implementation guarantees lockcnt->refcnt == 0 or 1 until unlocked
if this returns 0.

However, the below one looks not guarantee it. Since lockref_add_unless
and spinlock are not done atomically, there is a chance for someone
to increment it right before locking.

Or, I missed something?

> +/**
> + * lockref_put_or_lock - Decrements count unless the count is <= 1
> + *			 otherwise, the lock will be taken
> + * @lockcnt: pointer to struct lockref structure
> + * Return: 1 if count updated successfully or 0 if count <= 1 and lock taken
> + */
> +int
> +lockref_put_or_lock(struct lockref *lockcnt)
> +{
> +	if (lockref_add_unless(lockcnt, -1, 1))
> +		return 1;
> +	spin_lock(&lockcnt->lock);
> +	return 0;
> +}

BTW, it looks that your dcache patch knows this and keeps double check for
the case of lockcnt->refcnt > 1 in dput().

Thank you,

-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx

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