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SubjectRe: [PATCH v3] f2fs: fix to avoid race during access gc_thread pointer
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On 2018/5/5 18:02, 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, use sb->s_umount to exclude those operations

I encounter deadlock with this patch

dump_stack+0x5f/0x86
__lock_acquire+0xff7/0x10e0
lock_acquire+0xae/0x220
down_read+0x38/0x60 try lock s_umount again
need_SSR+0x5d/0x160 [f2fs]
allocate_segment_by_default+0xb7/0x1c0 [f2fs]
allocate_data_block+0x183/0x4c0 [f2fs]
do_write_page+0x52/0x80 [f2fs]
write_data_page+0x4a/0xd0 [f2fs]
do_write_data_page+0x327/0x630 [f2fs]
__write_data_page+0x34b/0x800 [f2fs]
__f2fs_write_data_pages+0x3f1/0x8e0 [f2fs]
f2fs_write_data_pages+0x27/0x30 [f2fs]
do_writepages+0x1a/0x70
__writeback_single_inode+0x55/0x7e0
writeback_sb_inodes+0x21b/0x490
__writeback_inodes_wb+0x7c/0xb0 trylock_super has alread hold s_umount
wb_writeback+0x3e2/0x580
wb_workfn+0x251/0x6b0
process_one_work+0x196/0x550
worker_thread+0x31/0x360
kthread+0xe3/0x110
ret_from_fork+0x2e/0x38

So, is it better to introduce private lock to avoid the race condition?

Thanks,

>
> Signed-off-by: Chao Yu <yuchao0@huawei.com>
> ---
> v3:
> - use sb->s_umount to make all race cases exclusive.
> fs/f2fs/gc.c | 4 ++++
> fs/f2fs/segment.c | 11 ++++++++++-
> fs/f2fs/sysfs.c | 14 +++++++++++---
> 3 files changed, 25 insertions(+), 4 deletions(-)
>
> diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> index 9bb2ddbbed1e..d7d469f9be0a 100644
> --- a/fs/f2fs/gc.c
> +++ b/fs/f2fs/gc.c
> @@ -187,17 +187,21 @@ 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 {
> + down_read(&sbi->sb->s_umount);
> p->gc_mode = select_gc_type(sbi->gc_thread, gc_type);
> + up_read(&sbi->sb->s_umount);
> 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 */
> + down_read(&sbi->sb->s_umount);
> if (gc_type != FG_GC &&
> (sbi->gc_thread && !sbi->gc_thread->gc_urgent) &&
> p->max_search > sbi->max_victim_search)
> p->max_search = sbi->max_victim_search;
> + up_read(&sbi->sb->s_umount);
>
> /* let's select beginning hot/small space first in no_heap mode*/
> if (test_opt(sbi, NOHEAP) &&
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index 320cc1c57246..74e184ab0544 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -174,11 +174,18 @@ bool need_SSR(struct f2fs_sb_info *sbi)
> int node_secs = get_blocktype_secs(sbi, F2FS_DIRTY_NODES);
> int dent_secs = get_blocktype_secs(sbi, F2FS_DIRTY_DENTS);
> int imeta_secs = get_blocktype_secs(sbi, F2FS_DIRTY_IMETA);
> + bool gc_urgent = false;
>
> if (test_opt(sbi, LFS))
> return false;
> +
> + down_read(&sbi->sb->s_umount);
> if (sbi->gc_thread && sbi->gc_thread->gc_urgent)
> - return true;
> + gc_urgent = true;
> + up_read(&sbi->sb->s_umount);
> +
> + if (gc_urgent)
> + return false;
>
> return free_sections(sbi) <= (node_secs + 2 * dent_secs + imeta_secs +
> SM_I(sbi)->min_ssr_sections + reserved_sections(sbi));
> @@ -1421,8 +1428,10 @@ static int issue_discard_thread(void *data)
> if (dcc->discard_wake)
> dcc->discard_wake = 0;
>
> + down_read(&sbi->sb->s_umount);
> if (sbi->gc_thread && sbi->gc_thread->gc_urgent)
> init_discard_policy(&dpolicy, DPOLICY_FORCE, 1);
> + up_read(&sbi->sb->s_umount);
>
> sb_start_intwrite(sbi->sb);
>
> diff --git a/fs/f2fs/sysfs.c b/fs/f2fs/sysfs.c
> index 6d8d8f41e517..1cba68812b32 100644
> --- a/fs/f2fs/sysfs.c
> +++ b/fs/f2fs/sysfs.c
> @@ -173,6 +173,7 @@ static ssize_t f2fs_sbi_store(struct f2fs_attr *a,
> unsigned long t;
> unsigned int *ui;
> ssize_t ret;
> + bool gc_entry = (a->struct_type == GC_THREAD);
>
> ptr = __struct_ptr(sbi, a->struct_type);
> if (!ptr)
> @@ -248,16 +249,23 @@ 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 (gc_entry)
> + down_read(&sbi->sb->s_umount);
> +
> 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);
> }
>
> + *ui = t;
> +
> + if (gc_entry)
> + up_read(&sbi->sb->s_umount);
> +
> return count;
> }
>
>

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