Re: [PATCH v4] f2fs: fix to avoid race during access gc_thread pointer

[Chao Yu]

On 2018/5/8 5:36, Jaegeuk Kim wrote:
> On 05/07, 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.
>>
>> In order to fix this issue, introduce gc_rwsem to exclude those operations.
>>
>> Signed-off-by: Chao Yu <yuchao0@xxxxxxxxxx>
>> ---
>> v4:
>> - use introduced sbi.gc_rwsem lock instead of sb.s_umount.
> 
> We can use this first.
> 
>>From e62e8d3ece6ee8a4aeac8ffd6161d25851f8b3f0 Mon Sep 17 00:00:00 2001
> From: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> Date: Mon, 7 May 2018 14:22:40 -0700
> Subject: [PATCH] f2fs: introduce sbi->gc_mode to determine the policy
> 
> This is to avoid sbi->gc_thread pointer access.
> 
> Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> ---
>  fs/f2fs/f2fs.h    |  8 ++++++++
>  fs/f2fs/gc.c      | 28 ++++++++++++----------------
>  fs/f2fs/gc.h      |  2 --
>  fs/f2fs/segment.c |  4 ++--
>  fs/f2fs/sysfs.c   | 33 +++++++++++++++++++++++++--------
>  5 files changed, 47 insertions(+), 28 deletions(-)
> 
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index 80490a7991a7..779d8b26878c 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -1065,6 +1065,13 @@ enum {
>  	MAX_TIME,
>  };
>  
> +enum {
> +	GC_NORMAL,
> +	GC_IDLE_CB,
> +	GC_IDLE_GREEDY,
> +	GC_URGENT,
> +};
> +
>  enum {
>  	WHINT_MODE_OFF,		/* not pass down write hints */
>  	WHINT_MODE_USER,	/* try to pass down hints given by users */
> @@ -1193,6 +1200,7 @@ struct f2fs_sb_info {
>  	struct mutex gc_mutex;			/* mutex for GC */
>  	struct f2fs_gc_kthread	*gc_thread;	/* GC thread */
>  	unsigned int cur_victim_sec;		/* current victim section num */
> +	unsigned int gc_mode;			/* current GC state */
>  
>  	/* threshold for gc trials on pinned files */
>  	u64 gc_pin_file_threshold;
> diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> index 9bb2ddbbed1e..7ec8ea75dfde 100644
> --- a/fs/f2fs/gc.c
> +++ b/fs/f2fs/gc.c
> @@ -76,7 +76,7 @@ static int gc_thread_func(void *data)
>  		 * invalidated soon after by user update or deletion.
>  		 * So, I'd like to wait some time to collect dirty segments.
>  		 */
> -		if (gc_th->gc_urgent) {
> +		if (sbi->gc_mode == GC_URGENT) {
>  			wait_ms = gc_th->urgent_sleep_time;
>  			mutex_lock(&sbi->gc_mutex);
>  			goto do_gc;
> @@ -131,8 +131,6 @@ int start_gc_thread(struct f2fs_sb_info *sbi)
>  	gc_th->max_sleep_time = DEF_GC_THREAD_MAX_SLEEP_TIME;
>  	gc_th->no_gc_sleep_time = DEF_GC_THREAD_NOGC_SLEEP_TIME;
>  
> -	gc_th->gc_idle = 0;
> -	gc_th->gc_urgent = 0;
>  	gc_th->gc_wake= 0;
>  
>  	sbi->gc_thread = gc_th;
> @@ -158,21 +156,19 @@ void stop_gc_thread(struct f2fs_sb_info *sbi)
>  	sbi->gc_thread = NULL;
>  }
>  
> -static int select_gc_type(struct f2fs_gc_kthread *gc_th, int gc_type)
> +static int select_gc_type(struct f2fs_sb_info *sbi, int gc_type)
>  {
>  	int gc_mode = (gc_type == BG_GC) ? GC_CB : GC_GREEDY;
>  
> -	if (!gc_th)
> -		return gc_mode;
> -
> -	if (gc_th->gc_idle) {
> -		if (gc_th->gc_idle == 1)
> -			gc_mode = GC_CB;
> -		else if (gc_th->gc_idle == 2)
> -			gc_mode = GC_GREEDY;
> -	}
> -	if (gc_th->gc_urgent)
> +	switch (sbi->gc_mode) {
> +	case GC_IDLE_CB:
> +		gc_mode = GC_CB;
> +		break;
> +	case GC_IDLE_GREEDY:
> +	case GC_URGENT:
>  		gc_mode = GC_GREEDY;
> +		break;
> +	}
>  	return gc_mode;
>  }
>  
> @@ -187,7 +183,7 @@ 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(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;
> @@ -195,7 +191,7 @@ static void select_policy(struct f2fs_sb_info *sbi, int gc_type,
>  
>  	/* 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) &&
> +			(sbi->gc_mode != GC_URGENT) &&
>  			p->max_search > sbi->max_victim_search)
>  		p->max_search = sbi->max_victim_search;
>  
> diff --git a/fs/f2fs/gc.h b/fs/f2fs/gc.h
> index b0045d4c8d1e..c8619e408009 100644
> --- a/fs/f2fs/gc.h
> +++ b/fs/f2fs/gc.h
> @@ -36,8 +36,6 @@ struct f2fs_gc_kthread {
>  	unsigned int no_gc_sleep_time;
>  
>  	/* for changing gc mode */
> -	unsigned int gc_idle;
> -	unsigned int gc_urgent;
>  	unsigned int gc_wake;
>  };
>  
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index 503a98abdb2f..5a7ed06291e0 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 (sbi->gc_mode == GC_URGENT)
>  		return true;
>  
>  	return free_sections(sbi) <= (node_secs + 2 * dent_secs + imeta_secs +
> @@ -1418,7 +1418,7 @@ 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 (sbi->gc_mode == GC_URGENT)
>  			init_discard_policy(&dpolicy, DPOLICY_FORCE, 1);
>  
>  		sb_start_intwrite(sbi->sb);
> diff --git a/fs/f2fs/sysfs.c b/fs/f2fs/sysfs.c
> index 6d8d8f41e517..dd940d156af6 100644
> --- a/fs/f2fs/sysfs.c
> +++ b/fs/f2fs/sysfs.c
> @@ -248,16 +248,33 @@ static ssize_t f2fs_sbi_store(struct f2fs_attr *a,
>  	if (!strcmp(a->attr.name, "trim_sections"))
>  		return -EINVAL;
>  
> +	if (!strcmp(a->attr.name, "gc_urgent")) {
> +		if (t >= 1) {
> +			sbi->gc_mode = GC_URGENT;
> +			if (sbi->gc_thread) {
> +				wake_up_interruptible_all(
> +					&sbi->gc_thread->gc_wait_queue_head);

F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info, gc_urgent, gc_mode);

We have moved gc_urgent into F2FS_SBI set, but still we will try to access
gc_wait_queue_head queue head after verifying sbi->gc_thread pointer.

So we need to add s_umount here to avoid racing with remount?

Thanks,

> +				wake_up_discard_thread(sbi, true);
> +			}
> +		} else {
> +			sbi->gc_mode = GC_NORMAL;
> +		}
> +		return count;
> +	}
> +	if (!strcmp(a->attr.name, "gc_idle")) {
> +		if (t == GC_IDLE_CB)
> +			sbi->gc_mode = GC_IDLE_CB;
> +		else if (t == GC_IDLE_GREEDY)
> +			sbi->gc_mode = GC_IDLE_GREEDY;
> +		else
> +			sbi->gc_mode = GC_NORMAL;
> +		return count;
> +	}
> +
>  	*ui = t;
>  
>  	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);
> -		wake_up_discard_thread(sbi, true);
> -	}
> -
>  	return count;
>  }
>  
> @@ -349,8 +366,8 @@ F2FS_RW_ATTR(GC_THREAD, f2fs_gc_kthread, gc_urgent_sleep_time,
>  F2FS_RW_ATTR(GC_THREAD, f2fs_gc_kthread, gc_min_sleep_time, min_sleep_time);
>  F2FS_RW_ATTR(GC_THREAD, f2fs_gc_kthread, gc_max_sleep_time, max_sleep_time);
>  F2FS_RW_ATTR(GC_THREAD, f2fs_gc_kthread, gc_no_gc_sleep_time, no_gc_sleep_time);
> -F2FS_RW_ATTR(GC_THREAD, f2fs_gc_kthread, gc_idle, gc_idle);
> -F2FS_RW_ATTR(GC_THREAD, f2fs_gc_kthread, gc_urgent, gc_urgent);
> +F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info, gc_idle, gc_mode);
> +F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info, gc_urgent, gc_mode);
>  F2FS_RW_ATTR(SM_INFO, f2fs_sm_info, reclaim_segments, rec_prefree_segments);
>  F2FS_RW_ATTR(DCC_INFO, discard_cmd_control, max_small_discards, max_discards);
>  F2FS_RW_ATTR(DCC_INFO, discard_cmd_control, discard_granularity, discard_granularity);
>