linux-kernel - Re: [PATCH v4 1/1] dcache: Translating dentry into pathname without taking rename

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Date:	Mon, 9 Sep 2013 18:29:05 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Waiman Long <Waiman.Long@...com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	George Spelvin <linux@...izon.com>,
	John Stoffel <john@...ffel.org>
Subject: Re: [PATCH v4 1/1] dcache: Translating dentry into pathname without
 taking rename_lock

On Mon, Sep 09, 2013 at 12:18:13PM -0400, Waiman Long wrote:
> +/**
> + * read_seqbegin_or_lock - begin a sequence number check or locking block
> + * lock: sequence lock
> + * seq : sequence number to be checked
> + *
> + * First try it once optimistically without taking the lock. If that fails,
> + * take the lock. The sequence number is also used as a marker for deciding
> + * whether to be a reader (even) or writer (odd).
> + * N.B. seq must be initialized to an even number to begin with.
> + */
> +static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
> +{
> +	if (!(*seq & 1)) {	/* Even */
> +		*seq = read_seqbegin(lock);
> +		rcu_read_lock();
> +	} else			/* Odd */
> +		write_seqlock(lock);
> +}

> +static inline int read_seqretry_or_unlock(seqlock_t *lock, int *seq)
> +{
> +	if (!(*seq & 1)) {	/* Even */
> +		rcu_read_unlock();
> +		if (read_seqretry(lock, *seq)) {
> +			(*seq)++;	/* Take writer lock */
> +			return 1;
> +		}
> +	} else			/* Odd */
> +		write_sequnlock(lock);
> +	return 0;
> +}

I'm not sure I like mixing rcu_read_lock() into that - d_path() and friends
can do that themselves just fine (it needs to be taken when seq is even),
and e.g. d_walk() doesn't need it at all.  Other than that, I'm OK with
this variant.
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