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Date: Fri, 05 Sep 2014 15:07:01 +0800
From: Chao Yu <chao2.yu@...sung.com>
To: 'Gu Zheng' <guz.fnst@...fujitsu.com>, jaegeuk@...nel.org
Cc: linux-kernel@...r.kernel.org,
linux-f2fs-devel@...ts.sourceforge.net
Subject: RE: [f2fs-dev] [PATCH] f2fs: use lock-less list(llist) to simplify the
flush cmd management
Hi Gu,
> -----Original Message-----
> From: Gu Zheng [mailto:guz.fnst@...fujitsu.com]
> Sent: Wednesday, September 03, 2014 5:16 PM
> To: jaegeuk@...nel.org
> Cc: Gu Zheng; linux-kernel@...r.kernel.org; linux-f2fs-devel@...ts.sourceforge.net
> 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@...fujitsu.com>
> ---
> 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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