Documentation/filesystems/Locking

   1         The text below describes the locking rules for VFS-related methods.
   2 It is (believed to be) up-to-date. *Please*, if you change anything in
   3 prototypes or locking protocols - update this file. And update the relevant
   4 instances in the tree, don't leave that to maintainers of filesystems/devices/
   5 etc. At the very least, put the list of dubious cases in the end of this file.
   6 Don't turn it into log - maintainers of out-of-the-tree code are supposed to
   7 be able to use diff(1).
   8         Thing currently missing here: socket operations. Alexey?
   9
  10 --------------------------- dentry_operations --------------------------
  11 prototypes:
  12         int (*d_revalidate)(struct dentry *, int);
  13         int (*d_hash) (struct dentry *, struct qstr *);
  14         int (*d_compare) (struct dentry *, struct qstr *, struct qstr *);
  15         int (*d_delete)(struct dentry *);
  16         void (*d_release)(struct dentry *);
  17         void (*d_iput)(struct dentry *, struct inode *);
  18         char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
  19
  20 locking rules:
  21         none have BKL
  22                 dcache_lock     rename_lock     ->d_lock        may block
  23 d_revalidate:   no              no              no              yes
  24 d_hash          no              no              no              yes
  25 d_compare:      no              yes             no              no
  26 d_delete:       yes             no              yes             no
  27 d_release:      no              no              no              yes
  28 d_iput:         no              no              no              yes
  29 d_dname:        no              no              no              no
  30
  31 --------------------------- inode_operations ---------------------------
  32 prototypes:
  33         int (*create) (struct inode *,struct dentry *,int, struct nameidata *);
  34         struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameid
  35 ata *);
  36         int (*link) (struct dentry *,struct inode *,struct dentry *);
  37         int (*unlink) (struct inode *,struct dentry *);
  38         int (*symlink) (struct inode *,struct dentry *,const char *);
  39         int (*mkdir) (struct inode *,struct dentry *,int);
  40         int (*rmdir) (struct inode *,struct dentry *);
  41         int (*mknod) (struct inode *,struct dentry *,int,dev_t);
  42         int (*rename) (struct inode *, struct dentry *,
  43                         struct inode *, struct dentry *);
  44         int (*readlink) (struct dentry *, char __user *,int);
  45         int (*follow_link) (struct dentry *, struct nameidata *);
  46         void (*truncate) (struct inode *);
  47         int (*permission) (struct inode *, int, struct nameidata *);
  48         int (*setattr) (struct dentry *, struct iattr *);
  49         int (*getattr) (struct vfsmount *, struct dentry *, struct kstat *);
  50         int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
  51         ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
  52         ssize_t (*listxattr) (struct dentry *, char *, size_t);
  53         int (*removexattr) (struct dentry *, const char *);
  54
  55 locking rules:
  56         all may block, none have BKL
  57                 i_mutex(inode)
  58 lookup:         yes
  59 create:         yes
  60 link:           yes (both)
  61 mknod:          yes
  62 symlink:        yes
  63 mkdir:          yes
  64 unlink:         yes (both)
  65 rmdir:          yes (both)      (see below)
  66 rename:         yes (all)       (see below)
  67 readlink:       no
  68 follow_link:    no
  69 truncate:       yes             (see below)
  70 setattr:        yes
  71 permission:     no
  72 getattr:        no
  73 setxattr:       yes
  74 getxattr:       no
  75 listxattr:      no
  76 removexattr:    yes
  77         Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_mutex on
  78 victim.
  79         cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem.
  80         ->truncate() is never called directly - it's a callback, not a
  81 method. It's called by vmtruncate() - library function normally used by
  82 ->setattr(). Locking information above applies to that call (i.e. is
  83 inherited from ->setattr() - vmtruncate() is used when ATTR_SIZE had been
  84 passed).
  85
  86 See Documentation/filesystems/directory-locking for more detailed discussion
  87 of the locking scheme for directory operations.
  88
  89 --------------------------- super_operations ---------------------------
  90 prototypes:
  91         struct inode *(*alloc_inode)(struct super_block *sb);
  92         void (*destroy_inode)(struct inode *);
  93         void (*read_inode) (struct inode *);
  94         void (*dirty_inode) (struct inode *);
  95         int (*write_inode) (struct inode *, int);
  96         void (*put_inode) (struct inode *);
  97         void (*drop_inode) (struct inode *);
  98         void (*delete_inode) (struct inode *);
  99         void (*put_super) (struct super_block *);
 100         void (*write_super) (struct super_block *);
 101         int (*sync_fs)(struct super_block *sb, int wait);
 102         void (*write_super_lockfs) (struct super_block *);
 103         void (*unlockfs) (struct super_block *);
 104         int (*statfs) (struct dentry *, struct kstatfs *);
 105         int (*remount_fs) (struct super_block *, int *, char *);
 106         void (*clear_inode) (struct inode *);
 107         void (*umount_begin) (struct super_block *);
 108         int (*show_options)(struct seq_file *, struct vfsmount *);
 109         ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
 110         ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
 111
 112 locking rules:
 113         All may block.
 114                         BKL     s_lock  s_umount
 115 alloc_inode:            no      no      no
 116 destroy_inode:          no
 117 read_inode:             no                              (see below)
 118 dirty_inode:            no                              (must not sleep)
 119 write_inode:            no
 120 put_inode:              no
 121 drop_inode:             no                              !!!inode_lock!!!
 122 delete_inode:           no
 123 put_super:              yes     yes     no
 124 write_super:            no      yes     read
 125 sync_fs:                no      no      read
 126 write_super_lockfs:     ?
 127 unlockfs:               ?
 128 statfs:                 no      no      no
 129 remount_fs:             yes     yes     maybe           (see below)
 130 clear_inode:            no
 131 umount_begin:           yes     no      no
 132 show_options:           no                              (vfsmount->sem)
 133 quota_read:             no      no      no              (see below)
 134 quota_write:            no      no      no              (see below)
 135
 136 ->read_inode() is not a method - it's a callback used in iget().
 137 ->remount_fs() will have the s_umount lock if it's already mounted.
 138 When called from get_sb_single, it does NOT have the s_umount lock.
 139 ->quota_read() and ->quota_write() functions are both guaranteed to
 140 be the only ones operating on the quota file by the quota code (via
 141 dqio_sem) (unless an admin really wants to screw up something and
 142 writes to quota files with quotas on). For other details about locking
 143 see also dquot_operations section.
 144
 145 --------------------------- file_system_type ---------------------------
 146 prototypes:
 147         int (*get_sb) (struct file_system_type *, int,
 148                        const char *, void *, struct vfsmount *);
 149         void (*kill_sb) (struct super_block *);
 150 locking rules:
 151                 may block       BKL
 152 get_sb          yes             yes
 153 kill_sb         yes             yes
 154
 155 ->get_sb() returns error or 0 with locked superblock attached to the vfsmount
 156 (exclusive on ->s_umount).
 157 ->kill_sb() takes a write-locked superblock, does all shutdown work on it,
 158 unlocks and drops the reference.
 159
 160 --------------------------- address_space_operations --------------------------
 161 prototypes:
 162         int (*writepage)(struct page *page, struct writeback_control *wbc);
 163         int (*readpage)(struct file *, struct page *);
 164         int (*sync_page)(struct page *);
 165         int (*writepages)(struct address_space *, struct writeback_control *);
 166         int (*set_page_dirty)(struct page *page);
 167         int (*readpages)(struct file *filp, struct address_space *mapping,
 168                         struct list_head *pages, unsigned nr_pages);
 169         int (*prepare_write)(struct file *, struct page *, unsigned, unsigned);
 170         int (*commit_write)(struct file *, struct page *, unsigned, unsigned);
 171         sector_t (*bmap)(struct address_space *, sector_t);
 172         int (*invalidatepage) (struct page *, unsigned long);
 173         int (*releasepage) (struct page *, int);
 174         int (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
 175                         loff_t offset, unsigned long nr_segs);
 176         int (*launder_page) (struct page *);
 177
 178 locking rules:
 179         All except set_page_dirty may block
 180
 181                         BKL     PageLocked(page)
 182 writepage:              no      yes, unlocks (see below)
 183 readpage:               no      yes, unlocks
 184 sync_page:              no      maybe
 185 writepages:             no
 186 set_page_dirty          no      no
 187 readpages:              no
 188 prepare_write:          no      yes
 189 commit_write:           no      yes
 190 bmap:                   yes
 191 invalidatepage:         no      yes
 192 releasepage:            no      yes
 193 direct_IO:              no
 194 launder_page:           no      yes
 195
 196         ->prepare_write(), ->commit_write(), ->sync_page() and ->readpage()
 197 may be called from the request handler (/dev/loop).
 198
 199         ->readpage() unlocks the page, either synchronously or via I/O
 200 completion.
 201
 202         ->readpages() populates the pagecache with the passed pages and starts
 203 I/O against them.  They come unlocked upon I/O completion.
 204
 205         ->writepage() is used for two purposes: for "memory cleansing" and for
 206 "sync".  These are quite different operations and the behaviour may differ
 207 depending upon the mode.
 208
 209 If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then
 210 it *must* start I/O against the page, even if that would involve
 211 blocking on in-progress I/O.
 212
 213 If writepage is called for memory cleansing (sync_mode ==
 214 WBC_SYNC_NONE) then its role is to get as much writeout underway as
 215 possible.  So writepage should try to avoid blocking against
 216 currently-in-progress I/O.
 217
 218 If the filesystem is not called for "sync" and it determines that it
 219 would need to block against in-progress I/O to be able to start new I/O
 220 against the page the filesystem should redirty the page with
 221 redirty_page_for_writepage(), then unlock the page and return zero.
 222 This may also be done to avoid internal deadlocks, but rarely.
 223
 224 If the filesytem is called for sync then it must wait on any
 225 in-progress I/O and then start new I/O.
 226
 227 The filesystem should unlock the page synchronously, before returning to the
 228 caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE
 229 value. WRITEPAGE_ACTIVATE means that page cannot really be written out
 230 currently, and VM should stop calling ->writepage() on this page for some
 231 time. VM does this by moving page to the head of the active list, hence the
 232 name.
 233
 234 Unless the filesystem is going to redirty_page_for_writepage(), unlock the page
 235 and return zero, writepage *must* run set_page_writeback() against the page,
 236 followed by unlocking it.  Once set_page_writeback() has been run against the
 237 page, write I/O can be submitted and the write I/O completion handler must run
 238 end_page_writeback() once the I/O is complete.  If no I/O is submitted, the
 239 filesystem must run end_page_writeback() against the page before returning from
 240 writepage.
 241
 242 That is: after 2.5.12, pages which are under writeout are *not* locked.  Note,
 243 if the filesystem needs the page to be locked during writeout, that is ok, too,
 244 the page is allowed to be unlocked at any point in time between the calls to
 245 set_page_writeback() and end_page_writeback().
 246
 247 Note, failure to run either redirty_page_for_writepage() or the combination of
 248 set_page_writeback()/end_page_writeback() on a page submitted to writepage
 249 will leave the page itself marked clean but it will be tagged as dirty in the
 250 radix tree.  This incoherency can lead to all sorts of hard-to-debug problems
 251 in the filesystem like having dirty inodes at umount and losing written data.
 252
 253         ->sync_page() locking rules are not well-defined - usually it is called
 254 with lock on page, but that is not guaranteed. Considering the currently
 255 existing instances of this method ->sync_page() itself doesn't look
 256 well-defined...
 257
 258         ->writepages() is used for periodic writeback and for syscall-initiated
 259 sync operations.  The address_space should start I/O against at least
 260 *nr_to_write pages.  *nr_to_write must be decremented for each page which is
 261 written.  The address_space implementation may write more (or less) pages
 262 than *nr_to_write asks for, but it should try to be reasonably close.  If
 263 nr_to_write is NULL, all dirty pages must be written.
 264
 265 writepages should _only_ write pages which are present on
 266 mapping->io_pages.
 267
 268         ->set_page_dirty() is called from various places in the kernel
 269 when the target page is marked as needing writeback.  It may be called
 270 under spinlock (it cannot block) and is sometimes called with the page
 271 not locked.
 272
 273         ->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
 274 filesystems and by the swapper. The latter will eventually go away. All
 275 instances do not actually need the BKL. Please, keep it that way and don't
 276 breed new callers.
 277
 278         ->invalidatepage() is called when the filesystem must attempt to drop
 279 some or all of the buffers from the page when it is being truncated.  It
 280 returns zero on success.  If ->invalidatepage is zero, the kernel uses
 281 block_invalidatepage() instead.
 282
 283         ->releasepage() is called when the kernel is about to try to drop the
 284 buffers from the page in preparation for freeing it.  It returns zero to
 285 indicate that the buffers are (or may be) freeable.  If ->releasepage is zero,
 286 the kernel assumes that the fs has no private interest in the buffers.
 287
 288         ->launder_page() may be called prior to releasing a page if
 289 it is still found to be dirty. It returns zero if the page was successfully
 290 cleaned, or an error value if not. Note that in order to prevent the page
 291 getting mapped back in and redirtied, it needs to be kept locked
 292 across the entire operation.
 293
 294         Note: currently almost all instances of address_space methods are
 295 using BKL for internal serialization and that's one of the worst sources
 296 of contention. Normally they are calling library functions (in fs/buffer.c)
 297 and pass foo_get_block() as a callback (on local block-based filesystems,
 298 indeed). BKL is not needed for library stuff and is usually taken by
 299 foo_get_block(). It's an overkill, since block bitmaps can be protected by
 300 internal fs locking and real critical areas are much smaller than the areas
 301 filesystems protect now.
 302
 303 ----------------------- file_lock_operations ------------------------------
 304 prototypes:
 305         void (*fl_insert)(struct file_lock *);  /* lock insertion callback */
 306         void (*fl_remove)(struct file_lock *);  /* lock removal callback */
 307         void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
 308         void (*fl_release_private)(struct file_lock *);
 309
 310
 311 locking rules:
 312                         BKL     may block
 313 fl_insert:              yes     no
 314 fl_remove:              yes     no
 315 fl_copy_lock:           yes     no
 316 fl_release_private:     yes     yes
 317
 318 ----------------------- lock_manager_operations ---------------------------
 319 prototypes:
 320         int (*fl_compare_owner)(struct file_lock *, struct file_lock *);
 321         void (*fl_notify)(struct file_lock *);  /* unblock callback */
 322         void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
 323         void (*fl_release_private)(struct file_lock *);
 324         void (*fl_break)(struct file_lock *); /* break_lease callback */
 325
 326 locking rules:
 327                         BKL     may block
 328 fl_compare_owner:       yes     no
 329 fl_notify:              yes     no
 330 fl_copy_lock:           yes     no
 331 fl_release_private:     yes     yes
 332 fl_break:               yes     no
 333
 334         Currently only NFSD and NLM provide instances of this class. None of the
 335 them block. If you have out-of-tree instances - please, show up. Locking
 336 in that area will change.
 337 --------------------------- buffer_head -----------------------------------
 338 prototypes:
 339         void (*b_end_io)(struct buffer_head *bh, int uptodate);
 340
 341 locking rules:
 342         called from interrupts. In other words, extreme care is needed here.
 343 bh is locked, but that's all warranties we have here. Currently only RAID1,
 344 highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices
 345 call this method upon the IO completion.
 346
 347 --------------------------- block_device_operations -----------------------
 348 prototypes:
 349         int (*open) (struct inode *, struct file *);
 350         int (*release) (struct inode *, struct file *);
 351         int (*ioctl) (struct inode *, struct file *, unsigned, unsigned long);
 352         int (*media_changed) (struct gendisk *);
 353         int (*revalidate_disk) (struct gendisk *);
 354
 355 locking rules:
 356                         BKL     bd_sem
 357 open:                   yes     yes
 358 release:                yes     yes
 359 ioctl:                  yes     no
 360 media_changed:          no      no
 361 revalidate_disk:        no      no
 362
 363 The last two are called only from check_disk_change().
 364
 365 --------------------------- file_operations -------------------------------
 366 prototypes:
 367         loff_t (*llseek) (struct file *, loff_t, int);
 368         ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
 369         ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
 370         ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
 371         ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
 372         int (*readdir) (struct file *, void *, filldir_t);
 373         unsigned int (*poll) (struct file *, struct poll_table_struct *);
 374         int (*ioctl) (struct inode *, struct file *, unsigned int,
 375                         unsigned long);
 376         long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
 377         long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
 378         int (*mmap) (struct file *, struct vm_area_struct *);
 379         int (*open) (struct inode *, struct file *);
 380         int (*flush) (struct file *);
 381         int (*release) (struct inode *, struct file *);
 382         int (*fsync) (struct file *, struct dentry *, int datasync);
 383         int (*aio_fsync) (struct kiocb *, int datasync);
 384         int (*fasync) (int, struct file *, int);
 385         int (*lock) (struct file *, int, struct file_lock *);
 386         ssize_t (*readv) (struct file *, const struct iovec *, unsigned long,
 387                         loff_t *);
 388         ssize_t (*writev) (struct file *, const struct iovec *, unsigned long,
 389                         loff_t *);
 390         ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t,
 391                         void __user *);
 392         ssize_t (*sendpage) (struct file *, struct page *, int, size_t,
 393                         loff_t *, int);
 394         unsigned long (*get_unmapped_area)(struct file *, unsigned long,
 395                         unsigned long, unsigned long, unsigned long);
 396         int (*check_flags)(int);
 397         int (*dir_notify)(struct file *, unsigned long);
 398 };
 399
 400 locking rules:
 401         All except ->poll() may block.
 402                         BKL
 403 llseek:                 no      (see below)
 404 read:                   no
 405 aio_read:               no
 406 write:                  no
 407 aio_write:              no
 408 readdir:                no
 409 poll:                   no
 410 ioctl:                  yes     (see below)
 411 unlocked_ioctl:         no      (see below)
 412 compat_ioctl:           no
 413 mmap:                   no
 414 open:                   maybe   (see below)
 415 flush:                  no
 416 release:                no
 417 fsync:                  no      (see below)
 418 aio_fsync:              no
 419 fasync:                 yes     (see below)
 420 lock:                   yes
 421 readv:                  no
 422 writev:                 no
 423 sendfile:               no
 424 sendpage:               no
 425 get_unmapped_area:      no
 426 check_flags:            no
 427 dir_notify:             no
 428
 429 ->llseek() locking has moved from llseek to the individual llseek
 430 implementations.  If your fs is not using generic_file_llseek, you
 431 need to acquire and release the appropriate locks in your ->llseek().
 432 For many filesystems, it is probably safe to acquire the inode
 433 semaphore.  Note some filesystems (i.e. remote ones) provide no
 434 protection for i_size so you will need to use the BKL.
 435
 436 ->open() locking is in-transit: big lock partially moved into the methods.
 437 The only exception is ->open() in the instances of file_operations that never
 438 end up in ->i_fop/->proc_fops, i.e. ones that belong to character devices
 439 (chrdev_open() takes lock before replacing ->f_op and calling the secondary
 440 method. As soon as we fix the handling of module reference counters all
 441 instances of ->open() will be called without the BKL.
 442
 443 Note: ext2_release() was *the* source of contention on fs-intensive
 444 loads and dropping BKL on ->release() helps to get rid of that (we still
 445 grab BKL for cases when we close a file that had been opened r/w, but that
 446 can and should be done using the internal locking with smaller critical areas).
 447 Current worst offender is ext2_get_block()...
 448
 449 ->fasync() is a mess. This area needs a big cleanup and that will probably
 450 affect locking.
 451
 452 ->readdir() and ->ioctl() on directories must be changed. Ideally we would
 453 move ->readdir() to inode_operations and use a separate method for directory
 454 ->ioctl() or kill the latter completely. One of the problems is that for
 455 anything that resembles union-mount we won't have a struct file for all
 456 components. And there are other reasons why the current interface is a mess...
 457
 458 ->ioctl() on regular files is superceded by the ->unlocked_ioctl() that
 459 doesn't take the BKL.
 460
 461 ->read on directories probably must go away - we should just enforce -EISDIR
 462 in sys_read() and friends.
 463
 464 ->fsync() has i_mutex on inode.
 465
 466 --------------------------- dquot_operations -------------------------------
 467 prototypes:
 468         int (*initialize) (struct inode *, int);
 469         int (*drop) (struct inode *);
 470         int (*alloc_space) (struct inode *, qsize_t, int);
 471         int (*alloc_inode) (const struct inode *, unsigned long);
 472         int (*free_space) (struct inode *, qsize_t);
 473         int (*free_inode) (const struct inode *, unsigned long);
 474         int (*transfer) (struct inode *, struct iattr *);
 475         int (*write_dquot) (struct dquot *);
 476         int (*acquire_dquot) (struct dquot *);
 477         int (*release_dquot) (struct dquot *);
 478         int (*mark_dirty) (struct dquot *);
 479         int (*write_info) (struct super_block *, int);
 480
 481 These operations are intended to be more or less wrapping functions that ensure
 482 a proper locking wrt the filesystem and call the generic quota operations.
 483
 484 What filesystem should expect from the generic quota functions:
 485
 486                 FS recursion    Held locks when called
 487 initialize:     yes             maybe dqonoff_sem
 488 drop:           yes             -
 489 alloc_space:    ->mark_dirty()  -
 490 alloc_inode:    ->mark_dirty()  -
 491 free_space:     ->mark_dirty()  -
 492 free_inode:     ->mark_dirty()  -
 493 transfer:       yes             -
 494 write_dquot:    yes             dqonoff_sem or dqptr_sem
 495 acquire_dquot:  yes             dqonoff_sem or dqptr_sem
 496 release_dquot:  yes             dqonoff_sem or dqptr_sem
 497 mark_dirty:     no              -
 498 write_info:     yes             dqonoff_sem
 499
 500 FS recursion means calling ->quota_read() and ->quota_write() from superblock
 501 operations.
 502
 503 ->alloc_space(), ->alloc_inode(), ->free_space(), ->free_inode() are called
 504 only directly by the filesystem and do not call any fs functions only
 505 the ->mark_dirty() operation.
 506
 507 More details about quota locking can be found in fs/dquot.c.
 508
 509 --------------------------- vm_operations_struct -----------------------------
 510 prototypes:
 511         void (*open)(struct vm_area_struct*);
 512         void (*close)(struct vm_area_struct*);
 513         int (*fault)(struct vm_area_struct*, struct vm_fault *);
 514         struct page *(*nopage)(struct vm_area_struct*, unsigned long, int *);
 515         int (*page_mkwrite)(struct vm_area_struct *, struct page *);
 516
 517 locking rules:
 518                 BKL     mmap_sem        PageLocked(page)
 519 open:           no      yes
 520 close:          no      yes
 521 fault:          no      yes
 522 nopage:         no      yes
 523 page_mkwrite:   no      yes             no
 524
 525         ->page_mkwrite() is called when a previously read-only page is
 526 about to become writeable. The file system is responsible for
 527 protecting against truncate races. Once appropriate action has been
 528 taking to lock out truncate, the page range should be verified to be
 529 within i_size. The page mapping should also be checked that it is not
 530 NULL.
 531
 532 ================================================================================
 533                         Dubious stuff
 534
 535 (if you break something or notice that it is broken and do not fix it yourself
 536 - at least put it here)
 537
 538 ipc/shm.c::shm_delete() - may need BKL.
 539 ->read() and ->write() in many drivers are (probably) missing BKL.
 540 drivers/sgi/char/graphics.c::sgi_graphics_nopage() - may need BKL.