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

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Date:	Mon, 09 Sep 2013 13:55:06 -0400
From:	Waiman Long <waiman.long@...com>
To:	Al Viro <viro@...IV.linux.org.uk>
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 09/09/2013 01:29 PM, Al Viro wrote:
> 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.

I think rcu_read_lock() is needed to make sure that the dentry won't be 
freed as we don't take d_lock now.

-Longman
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