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Date:	Mon, 10 Mar 2014 13:23:51 +0800
From:	Gu Zheng <guz.fnst@...fujitsu.com>
To:	Changman Lee <cm224.lee@...sung.com>
CC:	Kim <jaegeuk.kim@...sung.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	f2fs <linux-f2fs-devel@...ts.sourceforge.net>
Subject: Re: [f2fs-dev] [PATCH 5/5] f2fs: add a wait queue to avoid unnecessary,
 build_free_nid

Hi Changman,
On 03/10/2014 12:09 PM, Changman Lee wrote:

> On 금, 2014-03-07 at 18:43 +0800, Gu Zheng wrote:
>> Previously, when we try to alloc free nid while the build free nid
>> is going, the allocer will be run into the flow that waiting for
>> "nm_i->build_lock", see following:
>> 	/* We should not use stale free nids created by build_free_nids */
>> ---->	if (nm_i->fcnt && !on_build_free_nids(nm_i)) {
>> 		f2fs_bug_on(list_empty(&nm_i->free_nid_list));
>> 		list_for_each(this, &nm_i->free_nid_list) {
>> 			i = list_entry(this, struct free_nid, list);
>> 			if (i->state == NID_NEW)
>> 				break;
>> 		}
>>
>> 		f2fs_bug_on(i->state != NID_NEW);
>> 		*nid = i->nid;
>> 		i->state = NID_ALLOC;
>> 		nm_i->fcnt--;
>> 		spin_unlock(&nm_i->free_nid_list_lock);
>> 		return true;
>> 	}
>> 	spin_unlock(&nm_i->free_nid_list_lock);
>>
>> 	/* Let's scan nat pages and its caches to get free nids */
>> ---->	mutex_lock(&nm_i->build_lock);
>> 	build_free_nids(sbi);
>> 	mutex_unlock(&nm_i->build_lock);
>> and this will cause another unnecessary building free nid if the current
>> building free nid job is done.
>> So here we introduce a wait_queue to avoid this issue.
>>
>> Signed-off-by: Gu Zheng <guz.fnst@...fujitsu.com>
>> ---
>>  fs/f2fs/f2fs.h |    1 +
>>  fs/f2fs/node.c |   10 +++++++++-
>>  2 files changed, 10 insertions(+), 1 deletions(-)
>>
>> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
>> index f845e92..7ae193e 100644
>> --- a/fs/f2fs/f2fs.h
>> +++ b/fs/f2fs/f2fs.h
>> @@ -256,6 +256,7 @@ struct f2fs_nm_info {
>>  	spinlock_t free_nid_list_lock;	/* protect free nid list */
>>  	unsigned int fcnt;		/* the number of free node id */
>>  	struct mutex build_lock;	/* lock for build free nids */
>> +	wait_queue_head_t build_wq;	/* wait queue for build free nids */
>>  
>>  	/* for checkpoint */
>>  	char *nat_bitmap;		/* NAT bitmap pointer */
>> diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
>> index 4b7861d..ab44711 100644
>> --- a/fs/f2fs/node.c
>> +++ b/fs/f2fs/node.c
>> @@ -1422,7 +1422,13 @@ retry:
>>  	spin_lock(&nm_i->free_nid_list_lock);
>>  
>>  	/* We should not use stale free nids created by build_free_nids */
>> -	if (nm_i->fcnt && !on_build_free_nids(nm_i)) {
>> +	if (on_build_free_nids(nm_i)) {
>> +		spin_unlock(&nm_i->free_nid_list_lock);
>> +		wait_event(nm_i->build_wq, !on_build_free_nids(nm_i));
>> +		goto retry;
>> +	}
>> +
> 
> It would be better moving spin_lock(free_nid_list_lock) here after
> removing above spin_unlock().

Agree. It's better to place spin_lock here to avoid needless lock protection.

Regards,
Gu

> 
>> +	if (nm_i->fcnt) {
>>  		f2fs_bug_on(list_empty(&nm_i->free_nid_list));
>>  		list_for_each(this, &nm_i->free_nid_list) {
>>  			i = list_entry(this, struct free_nid, list);
>> @@ -1443,6 +1449,7 @@ retry:
>>  	mutex_lock(&nm_i->build_lock);
>>  	build_free_nids(sbi);
>>  	mutex_unlock(&nm_i->build_lock);
>> +	wake_up_all(&nm_i->build_wq);
>>  	goto retry;
>>  }
>>  
>> @@ -1813,6 +1820,7 @@ static int init_node_manager(struct f2fs_sb_info *sbi)
>>  	INIT_LIST_HEAD(&nm_i->dirty_nat_entries);
>>  
>>  	mutex_init(&nm_i->build_lock);
>> +	init_waitqueue_head(&nm_i->build_wq);
>>  	spin_lock_init(&nm_i->free_nid_list_lock);
>>  	rwlock_init(&nm_i->nat_tree_lock);
>>  
> 
> 
> 


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