Date:   Fri, 27 Apr 2018 19:37:51 -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 v2] f2fs: fix to avoid race during access gc_thread
 pointer

On 04/27, Chao Yu wrote:
> On 2018/4/27 10:04, Chao Yu wrote:
> > On 2018/4/27 0:03, Jaegeuk Kim wrote:
> >> On 04/24, 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
> >>>
> >>> 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.
> >>
> >> Do we just need to check &sb->s_umount in f2fs_sbi_store() and
> > 
> > Better,
> > 
> >> issue_discard_thread?
> > 
> > There is more cases can be called from different scenarios:
> > - select_policy()
> > - need_SSR()
> 
> BTW, do you agree with introducing {init,destroy}_gc_context as the patch does?

I don't think so. The issue only requires mutex() in sysfs/discard_thread.

> 
> Thanks,
> 
> > 
> > Thanks,
> > 
> >>
> >>>
> >>> In order to fix this issue, keep gc_thread structure valid in sbi all
> >>> the time instead of alloc/free it dynamically.
> >>>
> >>> In addition, add a rw semaphore to exclude rw operation in fields of
> >>> gc_thread.
> >>>
> >>> Signed-off-by: Chao Yu <yuchao0@...wei.com>
> >>> ---
> >>> v2:
> >>> - add a rw semaphore to exclude rw operation suggested by Jaegeuk.
> >>>  fs/f2fs/debug.c   |  3 +--
> >>>  fs/f2fs/f2fs.h    |  9 ++++++-
> >>>  fs/f2fs/gc.c      | 78 ++++++++++++++++++++++++++++++++++++-------------------
> >>>  fs/f2fs/gc.h      |  1 +
> >>>  fs/f2fs/segment.c | 10 +++++--
> >>>  fs/f2fs/super.c   | 26 +++++++++++++------
> >>>  fs/f2fs/sysfs.c   | 26 ++++++++++++++-----
> >>>  7 files changed, 107 insertions(+), 46 deletions(-)
> >>>
> >>> diff --git a/fs/f2fs/debug.c b/fs/f2fs/debug.c
> >>> index a66107b5cfff..0fbd674c66fb 100644
> >>> --- a/fs/f2fs/debug.c
> >>> +++ b/fs/f2fs/debug.c
> >>> @@ -221,8 +221,7 @@ 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 8f3ad9662d13..75d3b4875429 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);
> >>> @@ -2954,8 +2959,10 @@ 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);
> >>> +bool stop_gc_thread(struct f2fs_sb_info *sbi);
> >>>  block_t start_bidx_of_node(unsigned int node_ofs, struct inode *inode);
> >>>  int f2fs_gc(struct f2fs_sb_info *sbi, bool sync, bool background,
> >>>  			unsigned int segno);
> >>> diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> >>> index 70418b34c5f6..2febb84b2fd6 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;
> >>> @@ -135,35 +133,59 @@ int start_gc_thread(struct f2fs_sb_info *sbi)
> >>>  	gc_th->gc_urgent = 0;
> >>>  	gc_th->gc_wake= 0;
> >>>  
> >>> +	init_rwsem(&gc_th->gc_rwsem);
> >>> +
> >>>  	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)
> >>> +bool 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);
> >>> +	bool stopped = false;
> >>> +
> >>> +	down_write(&gc_th->gc_rwsem);
> >>> +	if (gc_th->f2fs_gc_task) {
> >>> +		kthread_stop(gc_th->f2fs_gc_task);
> >>> +		gc_th->f2fs_gc_task = NULL;
> >>> +		stopped = true;
> >>> +	}
> >>> +	up_write(&gc_th->gc_rwsem);
> >>> +
> >>> +	return stopped;
> >>>  }
> >>>  
> >>>  static int select_gc_type(struct f2fs_gc_kthread *gc_th, int gc_type)
> >>>  {
> >>>  	int gc_mode = (gc_type == BG_GC) ? GC_CB : GC_GREEDY;
> >>>  
> >>> -	if (!gc_th)
> >>> -		return gc_mode;
> >>> +	down_read(&gc_th->gc_rwsem);
> >>> +	if (!gc_th->f2fs_gc_task)
> >>> +		goto out;
> >>>  
> >>>  	if (gc_th->gc_idle) {
> >>>  		if (gc_th->gc_idle == 1)
> >>> @@ -173,6 +195,8 @@ static int select_gc_type(struct f2fs_gc_kthread *gc_th, int gc_type)
> >>>  	}
> >>>  	if (gc_th->gc_urgent)
> >>>  		gc_mode = GC_GREEDY;
> >>> +out:
> >>> +	up_read(&gc_th->gc_rwsem);
> >>>  	return gc_mode;
> >>>  }
> >>>  
> >>> @@ -187,17 +211,19 @@ 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) &&
> >>> +	down_read(&GC_I(sbi)->gc_rwsem);
> >>> +	if (gc_type != FG_GC && GC_I(sbi)->f2fs_gc_task &&
> >>> +			!GC_I(sbi)->gc_urgent &&
> >>>  			p->max_search > sbi->max_victim_search)
> >>>  		p->max_search = sbi->max_victim_search;
> >>> +	up_read(&GC_I(sbi)->gc_rwsem);
> >>>  
> >>>  	/* let's select beginning hot/small space first in no_heap mode*/
> >>>  	if (test_opt(sbi, NOHEAP) &&
> >>> diff --git a/fs/f2fs/gc.h b/fs/f2fs/gc.h
> >>> index b0045d4c8d1e..9a5b273328c2 100644
> >>> --- a/fs/f2fs/gc.h
> >>> +++ b/fs/f2fs/gc.h
> >>> @@ -26,6 +26,7 @@
> >>>  #define DEF_MAX_VICTIM_SEARCH 4096 /* covers 8GB */
> >>>  
> >>>  struct f2fs_gc_kthread {
> >>> +	struct rw_semaphore gc_rwsem;		/* semaphore for f2fs_gc_task */
> >>>  	struct task_struct *f2fs_gc_task;
> >>>  	wait_queue_head_t gc_wait_queue_head;
> >>>  
> >>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> >>> index 5960768d26df..f787eea1b2f6 100644
> >>> --- a/fs/f2fs/segment.c
> >>> +++ b/fs/f2fs/segment.c
> >>> @@ -177,8 +177,12 @@ bool need_SSR(struct f2fs_sb_info *sbi)
> >>>  
> >>>  	if (test_opt(sbi, LFS))
> >>>  		return false;
> >>> -	if (sbi->gc_thread && sbi->gc_thread->gc_urgent)
> >>> +	down_read(&GC_I(sbi)->gc_rwsem);
> >>> +	if (GC_I(sbi)->f2fs_gc_task && GC_I(sbi)->gc_urgent) {
> >>> +		down_read(&GC_I(sbi)->gc_rwsem);
> >>>  		return true;
> >>> +	}
> >>> +	up_read(&GC_I(sbi)->gc_rwsem);
> >>>  
> >>>  	return free_sections(sbi) <= (node_secs + 2 * dent_secs + imeta_secs +
> >>>  			SM_I(sbi)->min_ssr_sections + reserved_sections(sbi));
> >>> @@ -1425,8 +1429,10 @@ static int issue_discard_thread(void *data)
> >>>  		if (dcc->discard_wake)
> >>>  			dcc->discard_wake = 0;
> >>>  
> >>> -		if (sbi->gc_thread && sbi->gc_thread->gc_urgent)
> >>> +		down_read(&GC_I(sbi)->gc_rwsem);
> >>> +		if (GC_I(sbi)->f2fs_gc_task && GC_I(sbi)->gc_urgent)
> >>>  			init_discard_policy(&dpolicy, DPOLICY_FORCE, 1);
> >>> +		up_read(&GC_I(sbi)->gc_rwsem);
> >>>  
> >>>  		sb_start_intwrite(sbi->sb);
> >>>  
> >>> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
> >>> index dedb8e50b440..199f29dce86d 100644
> >>> --- a/fs/f2fs/super.c
> >>> +++ b/fs/f2fs/super.c
> >>> @@ -1044,6 +1044,7 @@ 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++)
> >>> @@ -1476,15 +1477,18 @@ 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) {
> >>> -			stop_gc_thread(sbi);
> >>> -			need_restart_gc = true;
> >>> +		need_restart_gc = stop_gc_thread(sbi);
> >>> +	} else {
> >>> +		down_write(&GC_I(sbi)->gc_rwsem);
> >>> +		if (!GC_I(sbi)->f2fs_gc_task) {
> >>> +			err = start_gc_thread(sbi);
> >>> +			if (err) {
> >>> +				up_write(&GC_I(sbi)->gc_rwsem);
> >>> +				goto restore_opts;
> >>> +			}
> >>> +			need_stop_gc = true;
> >>>  		}
> >>> -	} else if (!sbi->gc_thread) {
> >>> -		err = start_gc_thread(sbi);
> >>> -		if (err)
> >>> -			goto restore_opts;
> >>> -		need_stop_gc = true;
> >>> +		up_write(&GC_I(sbi)->gc_rwsem);
> >>>  	}
> >>>  
> >>>  	if (*flags & SB_RDONLY ||
> >>> @@ -2771,6 +2775,10 @@ 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 +2989,8 @@ 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..b8d09935e43f 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)
> >>> @@ -248,16 +248,28 @@ static ssize_t f2fs_sbi_store(struct f2fs_attr *a,
> >>>  	if (!strcmp(a->attr.name, "trim_sections"))
> >>>  		return -EINVAL;
> >>>  
> >>> -	*ui = t;
> >>> -
> >>> -	if (!strcmp(a->attr.name, "iostat_enable") && *ui == 0)
> >>> +	if (!strcmp(a->attr.name, "iostat_enable") && t == 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 (a->struct_type == GC_THREAD) {
> >>> +		down_write(&GC_I(sbi)->gc_rwsem);
> >>> +		if (!GC_I(sbi)->f2fs_gc_task) {
> >>> +			up_write(&GC_I(sbi)->gc_rwsem);
> >>> +			return -EINVAL;
> >>> +		}
> >>> +	}
> >>> +
> >>> +	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);
> >>>  	}
> >>>  
> >>> +	*ui = t;
> >>> +
> >>> +	if (a->struct_type == GC_THREAD)
> >>> +		up_write(&GC_I(sbi)->gc_rwsem);
> >>> +
> >>>  	return count;
> >>>  }
> >>>  
> >>> -- 
> >>> 2.15.0.55.gc2ece9dc4de6
> >>
> >> .
> >>
> > 
> > 
> > .
> > 

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