Date:   Thu, 19 Apr 2018 20:54:12 -0700
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <yuchao0@...wei.com>
Cc:     linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, chao@...nel.org
Subject: Re: [PATCH 5/5] f2fs: fix to avoid race during access gc_thread
 pointer

On 04/20, Chao Yu wrote:
> On 2018/4/20 11:19, Jaegeuk Kim wrote:
> > On 04/18, Chao Yu wrote:
> >> Thread A			Thread B		Thread C
> >> - f2fs_remount
> >>  - stop_gc_thread
> >> 				- f2fs_sbi_store
> >> 							- issue_discard_thread
> >>    sbi->gc_thread = NULL;
> >> 				  sbi->gc_thread->gc_wake = 1
> >> 							  access sbi->gc_thread->gc_urgent
> > 
> > Do we simply need a lock for this?
> 
> Code will be more complicated for handling existed and new coming fields with
> the sbi->gc_thread pointer, and causing unneeded lock overhead, right?
> 
> So let's just allocate memory during fill_super?

No, the case is when stopping the thread. We can keep the gc_thread and indicate
its state as "disabled". Then, we need to handle other paths with the state?

> 
> Thanks,
> 
> > 
> >>
> >> Previously, we allocate memory for sbi->gc_thread based on background
> >> gc thread mount option, the memory can be released if we turn off
> >> that mount option, but still there are several places access gc_thread
> >> pointer without considering race condition, result in NULL point
> >> dereference.
> >>
> >> In order to fix this issue, keep gc_thread structure valid in sbi all
> >> the time instead of alloc/free it dynamically.
> >>
> >> Signed-off-by: Chao Yu <yuchao0@...wei.com>
> >> ---
> >>  fs/f2fs/debug.c   |  3 +--
> >>  fs/f2fs/f2fs.h    |  7 +++++++
> >>  fs/f2fs/gc.c      | 58 +++++++++++++++++++++++++++++++++----------------------
> >>  fs/f2fs/segment.c |  4 ++--
> >>  fs/f2fs/super.c   | 13 +++++++++++--
> >>  fs/f2fs/sysfs.c   |  8 ++++----
> >>  6 files changed, 60 insertions(+), 33 deletions(-)
> >>
> >> diff --git a/fs/f2fs/debug.c b/fs/f2fs/debug.c
> >> index 715beb85e9db..7bb036a3bb81 100644
> >> --- a/fs/f2fs/debug.c
> >> +++ b/fs/f2fs/debug.c
> >> @@ -223,8 +223,7 @@ static void update_mem_info(struct f2fs_sb_info *sbi)
> >>  	si->cache_mem = 0;
> >>  
> >>  	/* build gc */
> >> -	if (sbi->gc_thread)
> >> -		si->cache_mem += sizeof(struct f2fs_gc_kthread);
> >> +	si->cache_mem += sizeof(struct f2fs_gc_kthread);
> >>  
> >>  	/* build merge flush thread */
> >>  	if (SM_I(sbi)->fcc_info)
> >> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> >> index 567c6bb57ae3..c553f63199e8 100644
> >> --- a/fs/f2fs/f2fs.h
> >> +++ b/fs/f2fs/f2fs.h
> >> @@ -1412,6 +1412,11 @@ static inline struct sit_info *SIT_I(struct f2fs_sb_info *sbi)
> >>  	return (struct sit_info *)(SM_I(sbi)->sit_info);
> >>  }
> >>  
> >> +static inline struct f2fs_gc_kthread *GC_I(struct f2fs_sb_info *sbi)
> >> +{
> >> +	return (struct f2fs_gc_kthread *)(sbi->gc_thread);
> >> +}
> >> +
> >>  static inline struct free_segmap_info *FREE_I(struct f2fs_sb_info *sbi)
> >>  {
> >>  	return (struct free_segmap_info *)(SM_I(sbi)->free_info);
> >> @@ -2954,6 +2959,8 @@ bool f2fs_overwrite_io(struct inode *inode, loff_t pos, size_t len);
> >>  /*
> >>   * gc.c
> >>   */
> >> +int init_gc_context(struct f2fs_sb_info *sbi);
> >> +void destroy_gc_context(struct f2fs_sb_info * sbi);
> >>  int start_gc_thread(struct f2fs_sb_info *sbi);
> >>  void stop_gc_thread(struct f2fs_sb_info *sbi);
> >>  block_t start_bidx_of_node(unsigned int node_ofs, struct inode *inode);
> >> diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> >> index da89ca16a55d..7d310e454b77 100644
> >> --- a/fs/f2fs/gc.c
> >> +++ b/fs/f2fs/gc.c
> >> @@ -26,8 +26,8 @@
> >>  static int gc_thread_func(void *data)
> >>  {
> >>  	struct f2fs_sb_info *sbi = data;
> >> -	struct f2fs_gc_kthread *gc_th = sbi->gc_thread;
> >> -	wait_queue_head_t *wq = &sbi->gc_thread->gc_wait_queue_head;
> >> +	struct f2fs_gc_kthread *gc_th = GC_I(sbi);
> >> +	wait_queue_head_t *wq = &gc_th->gc_wait_queue_head;
> >>  	unsigned int wait_ms;
> >>  
> >>  	wait_ms = gc_th->min_sleep_time;
> >> @@ -114,17 +114,15 @@ static int gc_thread_func(void *data)
> >>  	return 0;
> >>  }
> >>  
> >> -int start_gc_thread(struct f2fs_sb_info *sbi)
> >> +int init_gc_context(struct f2fs_sb_info *sbi)
> >>  {
> >>  	struct f2fs_gc_kthread *gc_th;
> >> -	dev_t dev = sbi->sb->s_bdev->bd_dev;
> >> -	int err = 0;
> >>  
> >>  	gc_th = f2fs_kmalloc(sbi, sizeof(struct f2fs_gc_kthread), GFP_KERNEL);
> >> -	if (!gc_th) {
> >> -		err = -ENOMEM;
> >> -		goto out;
> >> -	}
> >> +	if (!gc_th)
> >> +		return -ENOMEM;
> >> +
> >> +	gc_th->f2fs_gc_task = NULL;
> >>  
> >>  	gc_th->urgent_sleep_time = DEF_GC_THREAD_URGENT_SLEEP_TIME;
> >>  	gc_th->min_sleep_time = DEF_GC_THREAD_MIN_SLEEP_TIME;
> >> @@ -139,26 +137,41 @@ int start_gc_thread(struct f2fs_sb_info *sbi)
> >>  	gc_th->atomic_file[FG_GC] = 0;
> >>  
> >>  	sbi->gc_thread = gc_th;
> >> -	init_waitqueue_head(&sbi->gc_thread->gc_wait_queue_head);
> >> -	sbi->gc_thread->f2fs_gc_task = kthread_run(gc_thread_func, sbi,
> >> +
> >> +	return 0;
> >> +}
> >> +
> >> +void destroy_gc_context(struct f2fs_sb_info *sbi)
> >> +{
> >> +	kfree(GC_I(sbi));
> >> +	sbi->gc_thread = NULL;
> >> +}
> >> +
> >> +int start_gc_thread(struct f2fs_sb_info *sbi)
> >> +{
> >> +	struct f2fs_gc_kthread *gc_th = GC_I(sbi);
> >> +	dev_t dev = sbi->sb->s_bdev->bd_dev;
> >> +	int err = 0;
> >> +
> >> +	init_waitqueue_head(&gc_th->gc_wait_queue_head);
> >> +	gc_th->f2fs_gc_task = kthread_run(gc_thread_func, sbi,
> >>  			"f2fs_gc-%u:%u", MAJOR(dev), MINOR(dev));
> >>  	if (IS_ERR(gc_th->f2fs_gc_task)) {
> >>  		err = PTR_ERR(gc_th->f2fs_gc_task);
> >> -		kfree(gc_th);
> >> -		sbi->gc_thread = NULL;
> >> +		gc_th->f2fs_gc_task = NULL;
> >>  	}
> >> -out:
> >> +
> >>  	return err;
> >>  }
> >>  
> >>  void stop_gc_thread(struct f2fs_sb_info *sbi)
> >>  {
> >> -	struct f2fs_gc_kthread *gc_th = sbi->gc_thread;
> >> -	if (!gc_th)
> >> -		return;
> >> -	kthread_stop(gc_th->f2fs_gc_task);
> >> -	kfree(gc_th);
> >> -	sbi->gc_thread = NULL;
> >> +	struct f2fs_gc_kthread *gc_th = GC_I(sbi);
> >> +
> >> +	if (gc_th->f2fs_gc_task) {
> >> +		kthread_stop(gc_th->f2fs_gc_task);
> >> +		gc_th->f2fs_gc_task = NULL;
> >> +	}
> >>  }
> >>  
> >>  static int select_gc_type(struct f2fs_gc_kthread *gc_th, int gc_type)
> >> @@ -190,15 +203,14 @@ static void select_policy(struct f2fs_sb_info *sbi, int gc_type,
> >>  		p->max_search = dirty_i->nr_dirty[type];
> >>  		p->ofs_unit = 1;
> >>  	} else {
> >> -		p->gc_mode = select_gc_type(sbi->gc_thread, gc_type);
> >> +		p->gc_mode = select_gc_type(GC_I(sbi), gc_type);
> >>  		p->dirty_segmap = dirty_i->dirty_segmap[DIRTY];
> >>  		p->max_search = dirty_i->nr_dirty[DIRTY];
> >>  		p->ofs_unit = sbi->segs_per_sec;
> >>  	}
> >>  
> >>  	/* we need to check every dirty segments in the FG_GC case */
> >> -	if (gc_type != FG_GC &&
> >> -			(sbi->gc_thread && !sbi->gc_thread->gc_urgent) &&
> >> +	if (gc_type != FG_GC && !GC_I(sbi)->gc_urgent &&
> >>  			p->max_search > sbi->max_victim_search)
> >>  		p->max_search = sbi->max_victim_search;
> >>  
> >> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> >> index 831cefa088bc..c869ce54be4d 100644
> >> --- a/fs/f2fs/segment.c
> >> +++ b/fs/f2fs/segment.c
> >> @@ -177,7 +177,7 @@ bool need_SSR(struct f2fs_sb_info *sbi)
> >>  
> >>  	if (test_opt(sbi, LFS))
> >>  		return false;
> >> -	if (sbi->gc_thread && sbi->gc_thread->gc_urgent)
> >> +	if (GC_I(sbi)->gc_urgent)
> >>  		return true;
> >>  
> >>  	return free_sections(sbi) <= (node_secs + 2 * dent_secs + imeta_secs +
> >> @@ -1422,7 +1422,7 @@ static int issue_discard_thread(void *data)
> >>  		if (dcc->discard_wake)
> >>  			dcc->discard_wake = 0;
> >>  
> >> -		if (sbi->gc_thread && sbi->gc_thread->gc_urgent)
> >> +		if (GC_I(sbi)->gc_urgent)
> >>  			init_discard_policy(&dpolicy, DPOLICY_FORCE, 1);
> >>  
> >>  		sb_start_intwrite(sbi->sb);
> >> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
> >> index d01c11f5e9c1..f3a5f463496f 100644
> >> --- a/fs/f2fs/super.c
> >> +++ b/fs/f2fs/super.c
> >> @@ -1012,6 +1012,8 @@ static void f2fs_put_super(struct super_block *sb)
> >>  		write_checkpoint(sbi, &cpc);
> >>  	}
> >>  
> >> +	destroy_gc_context(sbi);
> >> +
> >>  	/* write_checkpoint can update stat informaion */
> >>  	f2fs_destroy_stats(sbi);
> >>  
> >> @@ -1044,6 +1046,7 @@ static void f2fs_put_super(struct super_block *sb)
> >>  	kfree(sbi->raw_super);
> >>  
> >>  	destroy_device_list(sbi);
> >> +	destroy_gc_context(sbi);
> >>  	mempool_destroy(sbi->write_io_dummy);
> >>  #ifdef CONFIG_QUOTA
> >>  	for (i = 0; i < MAXQUOTAS; i++)
> >> @@ -1476,11 +1479,11 @@ static int f2fs_remount(struct super_block *sb, int *flags, char *data)
> >>  	 * option. Also sync the filesystem.
> >>  	 */
> >>  	if ((*flags & SB_RDONLY) || !test_opt(sbi, BG_GC)) {
> >> -		if (sbi->gc_thread) {
> >> +		if (GC_I(sbi)->f2fs_gc_task) {
> >>  			stop_gc_thread(sbi);
> >>  			need_restart_gc = true;
> >>  		}
> >> -	} else if (!sbi->gc_thread) {
> >> +	} else if (!GC_I(sbi)->f2fs_gc_task) {
> >>  		err = start_gc_thread(sbi);
> >>  		if (err)
> >>  			goto restore_opts;
> >> @@ -2771,6 +2774,10 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
> >>  		goto free_meta_inode;
> >>  	}
> >>  
> >> +	err = init_gc_context(sbi);
> >> +	if (err)
> >> +		goto free_checkpoint;
> >> +
> >>  	/* Initialize device list */
> >>  	err = f2fs_scan_devices(sbi);
> >>  	if (err) {
> >> @@ -2981,6 +2988,8 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
> >>  	destroy_segment_manager(sbi);
> >>  free_devices:
> >>  	destroy_device_list(sbi);
> >> +	destroy_gc_context(sbi);
> >> +free_checkpoint:
> >>  	kfree(sbi->ckpt);
> >>  free_meta_inode:
> >>  	make_bad_inode(sbi->meta_inode);
> >> diff --git a/fs/f2fs/sysfs.c b/fs/f2fs/sysfs.c
> >> index 2c53de9251be..fb3cd477d985 100644
> >> --- a/fs/f2fs/sysfs.c
> >> +++ b/fs/f2fs/sysfs.c
> >> @@ -46,7 +46,7 @@ struct f2fs_attr {
> >>  static unsigned char *__struct_ptr(struct f2fs_sb_info *sbi, int struct_type)
> >>  {
> >>  	if (struct_type == GC_THREAD)
> >> -		return (unsigned char *)sbi->gc_thread;
> >> +		return (unsigned char *)GC_I(sbi);
> >>  	else if (struct_type == SM_INFO)
> >>  		return (unsigned char *)SM_I(sbi);
> >>  	else if (struct_type == DCC_INFO)
> >> @@ -252,9 +252,9 @@ static ssize_t f2fs_sbi_store(struct f2fs_attr *a,
> >>  
> >>  	if (!strcmp(a->attr.name, "iostat_enable") && *ui == 0)
> >>  		f2fs_reset_iostat(sbi);
> >> -	if (!strcmp(a->attr.name, "gc_urgent") && t == 1 && sbi->gc_thread) {
> >> -		sbi->gc_thread->gc_wake = 1;
> >> -		wake_up_interruptible_all(&sbi->gc_thread->gc_wait_queue_head);
> >> +	if (!strcmp(a->attr.name, "gc_urgent") && t == 1) {
> >> +		GC_I(sbi)->gc_wake = 1;
> >> +		wake_up_interruptible_all(&GC_I(sbi)->gc_wait_queue_head);
> >>  		wake_up_discard_thread(sbi, true);
> >>  	}
> >>  
> >> -- 
> >> 2.15.0.55.gc2ece9dc4de6
> > 
> > .
> > 

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