f2fs: avoid potential deadlock in f2fs_sbi_store

[  155.018460] ======================================================
[  155.021431] WARNING: possible circular locking dependency detected
[  155.024339] 4.18.0-rc3+ #5 Tainted: G           OE
[  155.026879] ------------------------------------------------------
[  155.029783] umount/2901 is trying to acquire lock:
[  155.032187] 00000000c4282f1f (kn->count#130){++++}, at: kernfs_remove+0x1f/0x30
[  155.035439]
[  155.035439] but task is already holding lock:
[  155.038892] 0000000056e4307b (&type->s_umount_key#41){++++}, at: deactivate_super+0x33/0x50
[  155.042602]
[  155.042602] which lock already depends on the new lock.
[  155.042602]
[  155.047465]
[  155.047465] the existing dependency chain (in reverse order) is:
[  155.051354]
[  155.051354] -> #1 (&type->s_umount_key#41){++++}:
[  155.054768]        f2fs_sbi_store+0x61/0x460 [f2fs]
[  155.057083]        kernfs_fop_write+0x113/0x1a0
[  155.059277]        __vfs_write+0x36/0x180
[  155.061250]        vfs_write+0xbe/0x1b0
[  155.063179]        ksys_write+0x55/0xc0
[  155.065068]        do_syscall_64+0x60/0x1b0
[  155.067071]        entry_SYSCALL_64_after_hwframe+0x49/0xbe
[  155.069529]
[  155.069529] -> #0 (kn->count#130){++++}:
[  155.072421]        __kernfs_remove+0x26f/0x2e0
[  155.074452]        kernfs_remove+0x1f/0x30
[  155.076342]        kobject_del.part.5+0xe/0x40
[  155.078354]        f2fs_put_super+0x12d/0x290 [f2fs]
[  155.080500]        generic_shutdown_super+0x6c/0x110
[  155.082655]        kill_block_super+0x21/0x50
[  155.084634]        kill_f2fs_super+0x9c/0xc0 [f2fs]
[  155.086726]        deactivate_locked_super+0x3f/0x70
[  155.088826]        cleanup_mnt+0x3b/0x70
[  155.090584]        task_work_run+0x93/0xc0
[  155.092367]        exit_to_usermode_loop+0xf0/0x100
[  155.094466]        do_syscall_64+0x162/0x1b0
[  155.096312]        entry_SYSCALL_64_after_hwframe+0x49/0xbe
[  155.098603]
[  155.098603] other info that might help us debug this:
[  155.098603]
[  155.102418]  Possible unsafe locking scenario:
[  155.102418]
[  155.105134]        CPU0                    CPU1
[  155.107037]        ----                    ----
[  155.108910]   lock(&type->s_umount_key#41);
[  155.110674]                                lock(kn->count#130);
[  155.113010]                                lock(&type->s_umount_key#41);
[  155.115608]   lock(kn->count#130);

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/sysfs.c

@@ -286,8 +286,10 @@ static ssize_t f2fs_sbi_store(struct f2fs_attr *a,
 	bool gc_entry = (!strcmp(a->attr.name, "gc_urgent") ||
 					a->struct_type == GC_THREAD);
 
-	if (gc_entry)
-		down_read(&sbi->sb->s_umount);
+	if (gc_entry) {
+		if (!down_read_trylock(&sbi->sb->s_umount))
+			return -EAGAIN;
+	}
 	ret = __sbi_store(a, sbi, buf, count);
 	if (gc_entry)
 		up_read(&sbi->sb->s_umount);