RE: [f2fs-dev] [PATCH] f2fs: use lock-less list(llist) to simplify the flush cmd management

[Chao Yu]

Hi Gu,

> -----Original Message-----
> From: Gu Zheng [mailto:guz.fnst@xxxxxxxxxxxxxx]
> Sent: Wednesday, September 03, 2014 5:16 PM
> To: jaegeuk@xxxxxxxxxx
> Cc: Gu Zheng; linux-kernel@xxxxxxxxxxxxxxx; linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx
> Subject: [f2fs-dev] [PATCH] f2fs: use lock-less list(llist) to simplify the flush cmd management
> 
> We use flush cmd control to collect many flush cmds, and flush them
> together. In this case, we use two list to manage the flush cmds
> (collect and dispatch), and one spin lock is used to protect this.
> In fact, the lock-less list(llist) is very suitable to this case,
> and we use simplify this routine.

Good catch for finding this scenario with llist!

> 
> Signed-off-by: Gu Zheng <guz.fnst@xxxxxxxxxxxxxx>
> ---
>  fs/f2fs/f2fs.h    |    7 +++----
>  fs/f2fs/segment.c |   31 +++++++++----------------------
>  2 files changed, 12 insertions(+), 26 deletions(-)
> 
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index e921242..1dd861c 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -334,16 +334,15 @@ enum {
>  struct flush_cmd {
>  	struct flush_cmd *next;

No needed here, we can remove it.

>  	struct completion wait;
> +	struct llist_node llnode;
>  	int ret;
>  };
> 
>  struct flush_cmd_control {
>  	struct task_struct *f2fs_issue_flush;	/* flush thread */
>  	wait_queue_head_t flush_wait_queue;	/* waiting queue for wake-up */
> -	struct flush_cmd *issue_list;		/* list for command issue */
> -	struct flush_cmd *dispatch_list;	/* list for command dispatch */
> -	spinlock_t issue_lock;			/* for issue list lock */
> -	struct flush_cmd *issue_tail;		/* list tail of issue list */
> +	struct llist_head issue_list;		/* list for command issue */
> +	struct llist_node *dispatch_list;	/* list for command dispatch */
>  };
> 
>  struct f2fs_sm_info {
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index 0aa337c..2db1f95 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -205,24 +205,19 @@ repeat:
>  	if (kthread_should_stop())
>  		return 0;
> 
> -	spin_lock(&fcc->issue_lock);
> -	if (fcc->issue_list) {
> -		fcc->dispatch_list = fcc->issue_list;
> -		fcc->issue_list = fcc->issue_tail = NULL;
> -	}
> -	spin_unlock(&fcc->issue_lock);
> -
> -	if (fcc->dispatch_list) {
> +	if (!llist_empty(&fcc->issue_list)) {
>  		struct bio *bio = bio_alloc(GFP_NOIO, 0);
> -		struct flush_cmd *cmd, *next;
> +		struct flush_cmd *cmd;
>  		int ret;
> 
> +		fcc->dispatch_list = llist_del_all(&fcc->issue_list);
> +		fcc->dispatch_list = llist_reverse_order(fcc->dispatch_list);
> +
>  		bio->bi_bdev = sbi->sb->s_bdev;
>  		ret = submit_bio_wait(WRITE_FLUSH, bio);
> 
> -		for (cmd = fcc->dispatch_list; cmd; cmd = next) {
> +		llist_for_each_entry(cmd, fcc->dispatch_list, llnode) {

Please use llist_for_each_entry_safe, Otherwise we might encounter oops as below:

[18727.381582] Call Trace:
[18727.381613]  [<c107c0bf>] __wake_up_locked+0x1f/0x30
[18727.381626]  [<c107c886>] complete+0x36/0x50
[18727.381645]  [<f0a01313>] issue_flush_thread+0x93/0x140 [f2fs]
[18727.381658]  [<c107c530>] ? add_wait_queue+0x50/0x50
[18727.381672]  [<f0a01280>] ? add_sit_entry+0xc0/0xc0 [f2fs]
[18727.381687]  [<c105fabb>] kthread+0x9b/0xb0
[18727.381701]  [<c159ad81>] ret_from_kernel_thread+0x21/0x30
[18727.381714]  [<c105fa20>] ? flush_kthread_worker+0x80/0x80

Thanks,
Yu

>  			cmd->ret = ret;
> -			next = cmd->next;
>  			complete(&cmd->wait);
>  		}
>  		bio_put(bio);
> @@ -230,7 +225,7 @@ repeat:
>  	}
> 
>  	wait_event_interruptible(*q,
> -			kthread_should_stop() || fcc->issue_list);
> +		kthread_should_stop() || !llist_empty(&fcc->issue_list));
>  	goto repeat;
>  }
> 
> @@ -249,17 +244,10 @@ int f2fs_issue_flush(struct f2fs_sb_info *sbi)
>  		return blkdev_issue_flush(sbi->sb->s_bdev, GFP_KERNEL, NULL);
> 
>  	init_completion(&cmd.wait);
> -	cmd.next = NULL;
> 
> -	spin_lock(&fcc->issue_lock);
> -	if (fcc->issue_list)
> -		fcc->issue_tail->next = &cmd;
> -	else
> -		fcc->issue_list = &cmd;
> -	fcc->issue_tail = &cmd;
> -	spin_unlock(&fcc->issue_lock);
> +	llist_add(&cmd.llnode, &fcc->issue_list);
> 
> -	if (!fcc->dispatch_list)
> +	if (fcc->dispatch_list == NULL)
>  		wake_up(&fcc->flush_wait_queue);
> 
>  	wait_for_completion(&cmd.wait);
> @@ -276,7 +264,6 @@ int create_flush_cmd_control(struct f2fs_sb_info *sbi)
>  	fcc = kzalloc(sizeof(struct flush_cmd_control), GFP_KERNEL);
>  	if (!fcc)
>  		return -ENOMEM;
> -	spin_lock_init(&fcc->issue_lock);
>  	init_waitqueue_head(&fcc->flush_wait_queue);
>  	SM_I(sbi)->cmd_control_info = fcc;
>  	fcc->f2fs_issue_flush = kthread_run(issue_flush_thread, sbi,
> --
> 1.7.7
> 
> 
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