Linux虛擬文件系統之文件系統卸載（sys_umount())

1. /*sys_umont系統調用*/
2. SYSCALL_DEFINE2(umount, char __user *, name, int, flags)
3. {
4. struct path path;
5. int retval;
6. /*找到裝載點的vfsmount實例和dentry實例，二者包裝
7. 在一個nameidata結構中*/
8. retval = user_path(name, &path);
9. if (retval)
10. goto out;
11. retval = -EINVAL;
12. /*如果查找的最終目錄不是文件系統的掛載點*/
13. if (path.dentry != path.mnt->mnt_root)
14. goto dput_and_out;
15. /*如果要卸載的文件系統還沒有安裝在命名空間中*/
16. if (!check_mnt(path.mnt))
17. goto dput_and_out;
19. retval = -EPERM;
20. /*如果用戶不具有卸載文件系統的特權*/
21. if (!capable(CAP_SYS_ADMIN))
22. goto dput_and_out;
23. /*實際umount工作*/
24. retval = do_umount(path.mnt, flags);
25. dput_and_out:
26. /* we mustn't call path_put() as that would clear mnt_expiry_mark */
27. dput(path.dentry);
28. mntput_no_expire(path.mnt);
29. out:
30. return retval;
31. }

卸載實際工作

1. static int do_umount(struct vfsmount *mnt, int flags)
2. {
3. /*從vfsmount對象的mnt_sb字段檢索超級塊對象sb的地址*/
4. struct super_block *sb = mnt->mnt_sb;
5. int retval;
6. /*初始化umount_list，該鏈表在後面的釋放中會做臨時鏈表
7. 用*/
8. LIST_HEAD(umount_list);
10. retval = security_sb_umount(mnt, flags);
11. if (retval)
12. return retval;
14. /*
15. * Allow userspace to request a mountpoint be expired rather than
16. * unmounting unconditionally. Unmount only happens if:
17. * (1) the mark is already set (the mark is cleared by mntput())
18. * (2) the usage count == 1 [parent vfsmount] + 1 [sys_umount]
19. */
20. /*如果設置了MNT_EXPIRE標志，即要標記掛載點“到期”*/
21. if (flags & MNT_EXPIRE) {
22. /*若要卸載的文件系統是根文件系統或者同時設置了
23. MNT_FORCE或MNT_DETACH，則返回-EINVAL*/
24. if (mnt == current->fs->root.mnt ||
25. flags & (MNT_FORCE | MNT_DETACH))
26. return -EINVAL;
27. /*檢查vfsmount的引用計數，若不為2，則返回-EBUSY，
28. 要卸載的文件系統在卸載的時候不能有引用者，
29. 這個2代表vfsmount的父vfsmount和sys_umount()對本對象的引用*/
30. if (atomic_read(&mnt->mnt_count) != 2)
31. return -EBUSY;
32. /*設置vfsmount對象的mnt_expiry_mark字段為1。*/
33. if (!xchg(&mnt->mnt_expiry_mark, 1))
34. return -EAGAIN;
35. }
37. /*
38. * If we may have to abort operations to get out of this
39. * mount, and they will themselves hold resources we must
40. * allow the fs to do things. In the Unix tradition of
41. * 'Gee thats tricky lets do it in userspace' the umount_begin
42. * might fail to complete on the first run through as other tasks
43. * must return, and the like. Thats for the mount program to worry
44. * about for the moment.
45. */
46. /*如果用戶要求強制卸載操作，則調用umount_begin
47. 超級塊操作中斷任何正在進行的安裝操作*/
48. /*當然如果特定的文件系統定義了下面函數則調用它*/
49. if (flags & MNT_FORCE && sb->s_op->umount_begin) {
50. sb->s_op->umount_begin(sb);
51. }
53. /*
54. * No sense to grab the lock for this test, but test itself looks
55. * somewhat bogus. Suggestions for better replacement?
56. * Ho-hum... In principle, we might treat that as umount + switch
57. * to rootfs. GC would eventually take care of the old vfsmount.
58. * Actually it makes sense, especially if rootfs would contain a
59. * /reboot - static binary that would close all descriptors and
60. * call reboot(9). Then init(8) could umount root and exec /reboot.
61. */
62. /*如果要卸載的文件系統是根文件系統，且用戶
63. 並不要求真正地把它卸載下來（即設置了MNT_DETACH標志，
64. 這個標志僅僅標記掛載點為不能再訪問，知道掛載不busy
65. 時才卸載），則調用do_remount_sb()重新安裝根文件系統為只
66. 讀並終止，並返回do_remount_sb()的返回值。*/
67. if (mnt == current->fs->root.mnt && !(flags & MNT_DETACH)) {
68. /*
69. * Special case for "unmounting" root ...
70. * we just try to remount it readonly.
71. */
72. down_write(&sb->s_umount);
73. if (!(sb->s_flags & MS_RDONLY))
74. retval = do_remount_sb(sb, MS_RDONLY, NULL, 0);
75. up_write(&sb->s_umount);
76. return retval;
77. }
79. down_write(&namespace_sem);
80. /*為進行寫操作而獲取當前進程的namespace_sem讀/寫信號量和vfsmount_lock自旋鎖*/
81. spin_unlock(&vfsmount_lock);
82. spin_lock(&vfsmount_lock);
83. event++;
85. if (!(flags & MNT_DETACH))
86. shrink_submounts(mnt, &umount_list);
88. retval = -EBUSY;
89. /*如果已安裝文件系統不包含任何子安裝文件系統的安裝點，或者用戶要求強制
90. 卸載文件系統，則調用umount_tree()卸載文件系統（及其所有子文件系統）。*/
91. if (flags & MNT_DETACH || !propagate_mount_busy(mnt, 2)) {
92. if (!list_empty(&mnt->mnt_list))
93. /*完成實際的底層的卸載文件系統的任務。首先他將mnt的所有孩子移動至kill鏈表中，
94. 也就是傳遞進去的umount_list，然後將kill鏈表中的所有的vfsmount對象的一些字段設為無效狀態。
95. */
96. umount_tree(mnt, 1, &umount_list);
97. retval = 0;
98. }
100. if (retval)
101. security_sb_umount_busy(mnt);
102. /*釋放vfsmount_lock自旋鎖和當前進程的namespace_sem讀/寫信號量*/
103. up_write(&namespace_sem);
104. /*減小相應文件系統根目錄的目錄項對象和已經安裝文件系統
105. 描述符的引用計數器值，這些計數器值由path_lookup()增加*/
106. release_mounts(&umount_list);
107. return retval;
108. }