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Date:   Fri, 27 Apr 2018 10:04:44 +0800
From:   Chao Yu <yuchao0@...wei.com>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
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 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()

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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