/* Unix SMB/CIFS implementation. Trusted domain names cache on top of gencache. Copyright (C) Rafal Szczesniak 2002 This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see . */ #include "includes.h" #undef DBGC_CLASS #define DBGC_CLASS DBGC_ALL /* there's no proper class yet */ #define TDOMKEY_FMT "TDOM/%s" #define TDOMTSKEY "TDOMCACHE/TIMESTAMP" /** * @file trustdom_cache.c * * Implementation of trusted domain names cache useful when * samba acts as domain member server. In such case, caching * domain names currently trusted gives a performance gain * because there's no need to query PDC each time we need * list of trusted domains **/ /** * Initialise trustdom name caching system. Call gencache * initialisation routine to perform necessary activities. * * @return true upon successful cache initialisation or * false if cache init failed **/ bool trustdom_cache_enable(void) { /* Init trustdom cache by calling gencache initialisation */ if (!gencache_init()) { DEBUG(2, ("trustdomcache_enable: Couldn't initialise trustdom cache on top of gencache.\n")); return False; } return True; } /** * Shutdown trustdom name caching system. Calls gencache * shutdown function. * * @return true upon successful cache close or * false if it failed **/ bool trustdom_cache_shutdown(void) { /* Close trustdom cache by calling gencache shutdown */ if (!gencache_shutdown()) { DEBUG(2, ("trustdomcache_shutdown: Couldn't shutdown trustdom cache on top of gencache.\n")); return False; } return True; } /** * Form up trustdom name key. It is based only * on domain name now. * * @param name trusted domain name * @return cache key for use in gencache mechanism **/ static char* trustdom_cache_key(const char* name) { char* keystr = NULL; asprintf_strupper_m(&keystr, TDOMKEY_FMT, name); return keystr; } /** * Store trusted domain in gencache as the domain name (key) * and trusted domain's SID (value) * * @param name trusted domain name * @param alt_name alternative trusted domain name (used in ADS domains) * @param sid trusted domain's SID * @param timeout cache entry expiration time * @return true upon successful value storing or * false if store attempt failed **/ bool trustdom_cache_store(char* name, char* alt_name, const DOM_SID *sid, time_t timeout) { char *key, *alt_key; fstring sid_string; bool ret; /* * we use gecache call to avoid annoying debug messages * about initialised trustdom */ if (!gencache_init()) return False; DEBUG(5, ("trustdom_store: storing SID %s of domain %s\n", sid_string_dbg(sid), name)); key = trustdom_cache_key(name); alt_key = alt_name ? trustdom_cache_key(alt_name) : NULL; /* Generate string representation domain SID */ sid_to_fstring(sid_string, sid); /* * try to put the names in the cache */ if (alt_key) { ret = gencache_set(alt_key, sid_string, timeout); if ( ret ) { ret = gencache_set(key, sid_string, timeout); } SAFE_FREE(alt_key); SAFE_FREE(key); return ret; } ret = gencache_set(key, sid_string, timeout); SAFE_FREE(key); return ret; } /** * Fetch trusted domain's SID from the gencache. * This routine can also be used to check whether given * domain is currently trusted one. * * @param name trusted domain name * @param sid trusted domain's SID to be returned * @return true if entry is found or * false if has expired/doesn't exist **/ bool trustdom_cache_fetch(const char* name, DOM_SID* sid) { char *key = NULL, *value = NULL; time_t timeout; /* init the cache */ if (!gencache_init()) return False; /* exit now if null pointers were passed as they're required further */ if (!sid) return False; /* prepare a key and get the value */ key = trustdom_cache_key(name); if (!key) return False; if (!gencache_get(key, &value, &timeout)) { DEBUG(5, ("no entry for trusted domain %s found.\n", name)); SAFE_FREE(key); return False; } else { SAFE_FREE(key); DEBUG(5, ("trusted domain %s found (%s)\n", name, value)); } /* convert sid string representation into DOM_SID structure */ if(! string_to_sid(sid, value)) { sid = NULL; SAFE_FREE(value); return False; } SAFE_FREE(value); return True; } /******************************************************************* fetch the timestamp from the last update *******************************************************************/ uint32 trustdom_cache_fetch_timestamp( void ) { char *value = NULL; time_t timeout; uint32 timestamp; /* init the cache */ if (!gencache_init()) return False; if (!gencache_get(TDOMTSKEY, &value, &timeout)) { DEBUG(5, ("no timestamp for trusted domain cache located.\n")); SAFE_FREE(value); return 0; } timestamp = atoi(value); SAFE_FREE(value); return timestamp; } /******************************************************************* store the timestamp from the last update *******************************************************************/ bool trustdom_cache_store_timestamp( uint32 t, time_t timeout ) { fstring value; /* init the cache */ if (!gencache_init()) return False; fstr_sprintf(value, "%d", t ); if (!gencache_set(TDOMTSKEY, value, timeout)) { DEBUG(5, ("failed to set timestamp for trustdom_cache\n")); return False; } return True; } /** * Delete single trustdom entry. Look at the * gencache_iterate definition. * **/ static void flush_trustdom_name(const char* key, const char *value, time_t timeout, void* dptr) { gencache_del(key); DEBUG(5, ("Deleting entry %s\n", key)); } /** * Flush all the trusted domains entries from the cache. **/ void trustdom_cache_flush(void) { if (!gencache_init()) return; /* * iterate through each TDOM cache's entry and flush it * by flush_trustdom_name function */ gencache_iterate(flush_trustdom_name, NULL, trustdom_cache_key("*")); DEBUG(5, ("Trusted domains cache flushed\n")); } /******************************************************************** update the trustdom_cache if needed ********************************************************************/ #define TRUSTDOM_UPDATE_INTERVAL 600 void update_trustdom_cache( void ) { char **domain_names; DOM_SID *dom_sids; uint32 num_domains; uint32 last_check; int time_diff; TALLOC_CTX *mem_ctx = NULL; time_t now = time(NULL); int i; /* get the timestamp. We have to initialise it if the last timestamp == 0 */ if ( (last_check = trustdom_cache_fetch_timestamp()) == 0 ) trustdom_cache_store_timestamp(0, now+TRUSTDOM_UPDATE_INTERVAL); time_diff = (int) (now - last_check); if ( (time_diff > 0) && (time_diff < TRUSTDOM_UPDATE_INTERVAL) ) { DEBUG(10,("update_trustdom_cache: not time to update trustdom_cache yet\n")); return; } /* note that we don't lock the timestamp. This prevents this smbd from blocking all other smbd daemons while we enumerate the trusted domains */ trustdom_cache_store_timestamp(now, now+TRUSTDOM_UPDATE_INTERVAL); if ( !(mem_ctx = talloc_init("update_trustdom_cache")) ) { DEBUG(0,("update_trustdom_cache: talloc_init() failed!\n")); goto done; } /* get the domains and store them */ if ( enumerate_domain_trusts(mem_ctx, lp_workgroup(), &domain_names, &num_domains, &dom_sids)) { for ( i=0; i 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 
/*
 * fs/fs-writeback.c
 *
 * Copyright (C) 2002, Linus Torvalds.
 *
 * Contains all the functions related to writing back and waiting
 * upon dirty inodes against superblocks, and writing back dirty
 * pages against inodes.  ie: data writeback.  Writeout of the
 * inode itself is not handled here.
 *
 * 10Apr2002	akpm@zip.com.au
 *		Split out of fs/inode.c
 *		Additions for address_space-based writeback
 */

#include <linux/kernel.h>
#include <linux/spinlock.h>
#include <linux/sched.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/writeback.h>
#include <linux/blkdev.h>
#include <linux/backing-dev.h>
#include <linux/buffer_head.h>
#include "internal.h"

/**
 *	__mark_inode_dirty -	internal function
 *	@inode: inode to mark
 *	@flags: what kind of dirty (i.e. I_DIRTY_SYNC)
 *	Mark an inode as dirty. Callers should use mark_inode_dirty or
 *  	mark_inode_dirty_sync.
 *
 * Put the inode on the super block's dirty list.
 *
 * CAREFUL! We mark it dirty unconditionally, but move it onto the
 * dirty list only if it is hashed or if it refers to a blockdev.
 * If it was not hashed, it will never be added to the dirty list
 * even if it is later hashed, as it will have been marked dirty already.
 *
 * In short, make sure you hash any inodes _before_ you start marking
 * them dirty.
 *
 * This function *must* be atomic for the I_DIRTY_PAGES case -
 * set_page_dirty() is called under spinlock in several places.
 *
 * Note that for blockdevs, inode->dirtied_when represents the dirtying time of
 * the block-special inode (/dev/hda1) itself.  And the ->dirtied_when field of
 * the kernel-internal blockdev inode represents the dirtying time of the
 * blockdev's pages.  This is why for I_DIRTY_PAGES we always use
 * page->mapping->host, so the page-dirtying time is recorded in the internal
 * blockdev inode.
 */
void __mark_inode_dirty(struct inode *inode, int flags)
{
	struct super_block *sb = inode->i_sb;

	/*
	 * Don't do this for I_DIRTY_PAGES - that doesn't actually
	 * dirty the inode itself
	 */
	if (flags & (I_DIRTY_SYNC | I_DIRTY_DATASYNC)) {
		if (sb->s_op->dirty_inode)
			sb->s_op->dirty_inode(inode);
	}

	/*
	 * make sure that changes are seen by all cpus before we test i_state
	 * -- mikulas
	 */
	smp_mb();

	/* avoid the locking if we can */
	if ((inode->i_state & flags) == flags)
		return;

	if (unlikely(block_dump)) {
		struct dentry *dentry = NULL;
		const char *name = "?";

		if (!list_empty(&inode->i_dentry)) {
			dentry = list_entry(inode->i_dentry.next,
					    struct dentry, d_alias);
			if (dentry && dentry->d_name.name)
				name = (const char *) dentry->d_name.name;
		}

		if (inode->i_ino || strcmp(inode->i_sb->s_id, "bdev"))
			printk(KERN_DEBUG
			       "%s(%d): dirtied inode %lu (%s) on %s\n",
			       current->comm, current->pid, inode->i_ino,
			       name, inode->i_sb->s_id);
	}

	spin_lock(&inode_lock);
	if ((inode->i_state & flags) != flags) {
		const int was_dirty = inode->i_state & I_DIRTY;

		inode->i_state |= flags;

		/*
		 * If the inode is locked, just update its dirty state. 
		 * The unlocker will place the inode on the appropriate
		 * superblock list, based upon its state.
		 */
		if (inode->i_state & I_LOCK)
			goto out;

		/*
		 * Only add valid (hashed) inodes to the superblock's
		 * dirty list.  Add blockdev inodes as well.
		 */
		if (!S_ISBLK(inode->i_mode)) {
			if (hlist_unhashed(&inode->i_hash))
				goto out;
		}
		if (inode->i_state & (I_FREEING|I_CLEAR))
			goto out;

		/*
		 * If the inode was already on s_dirty or s_io, don't
		 * reposition it (that would break s_dirty time-ordering).
		 */
		if (!was_dirty) {
			inode->dirtied_when = jiffies;
			list_move(&inode->i_list, &sb->s_dirty);
		}
	}
out:
	spin_unlock(&inode_lock);
}

EXPORT_SYMBOL(__mark_inode_dirty);

static int write_inode(struct inode *inode, int sync)
{
	if (inode->i_sb->s_op->write_inode && !is_bad_inode(inode))
		return inode->i_sb->s_op->write_inode(inode, sync);
	return 0;
}

/*
 * Write a single inode's dirty pages and inode data out to disk.
 * If `wait' is set, wait on the writeout.
 *
 * The whole writeout design is quite complex and fragile.  We want to avoid
 * starvation of particular inodes when others are being redirtied, prevent
 * livelocks, etc.
 *
 * Called under inode_lock.
 */
static int
__sync_single_inode(struct inode *inode, struct writeback_control *wbc)
{
	unsigned dirty;
	struct address_space *mapping = inode->i_mapping;
	struct super_block *sb = inode->i_sb;
	int wait = wbc->sync_mode == WB_SYNC_ALL;
	int ret;

	BUG_ON(inode->i_state & I_LOCK);

	/* Set I_LOCK, reset I_DIRTY */
	dirty = inode->i_state & I_DIRTY;
	inode->i_state |= I_LOCK;
	inode->i_state &= ~I_DIRTY;

	spin_unlock(&inode_lock);

	ret = do_writepages(mapping, wbc);

	/* Don't write the inode if only I_DIRTY_PAGES was set */
	if (dirty & (I_DIRTY_SYNC | I_DIRTY_DATASYNC)) {
		int err = write_inode(inode, wait);
		if (ret == 0)
			ret = err;
	}

	if (wait) {
		int err = filemap_fdatawait(mapping);
		if (ret == 0)
			ret = err;
	}

	spin_lock(&inode_lock);
	inode->i_state &= ~I_LOCK;
	if (!(inode->i_state & I_FREEING)) {
		if (!(inode->i_state & I_DIRTY) &&
		    mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
			/*
			 * We didn't write back all the pages.  nfs_writepages()
			 * sometimes bales out without doing anything. Redirty
			 * the inode.  It is still on sb->s_io.
			 */
			if (wbc->for_kupdate) {
				/*
				 * For the kupdate function we leave the inode
				 * at the head of sb_dirty so it will get more
				 * writeout as soon as the queue becomes
				 * uncongested.
				 */
				inode->i_state |= I_DIRTY_PAGES;
				list_move_tail(&inode->i_list, &sb->s_dirty);
			} else {
				/*
				 * Otherwise fully redirty the inode so that
				 * other inodes on this superblock will get some
				 * writeout.  Otherwise heavy writing to one
				 * file would indefinitely suspend writeout of
				 * all the other files.
				 */
				inode->i_state |= I_DIRTY_PAGES;
				inode->dirtied_when = jiffies;
				list_move(&inode->i_list, &sb->s_dirty);
			}
		} else if (inode->i_state & I_DIRTY) {
			/*
			 * Someone redirtied the inode while were writing back
			 * the pages.
			 */
			list_move(&inode->i_list, &sb->s_dirty);
		} else if (atomic_read(&inode->i_count)) {
			/*
			 * The inode is clean, inuse
			 */
			list_move(&inode->i_list, &inode_in_use);
		} else {
			/*
			 * The inode is clean, unused
			 */
			list_move(&inode->i_list, &inode_unused);
		}
	}
	wake_up_inode(inode);
	return ret;
}

/*
 * Write out an inode's dirty pages.  Called under inode_lock.  Either the
 * caller has ref on the inode (either via __iget or via syscall against an fd)
 * or the inode has I_WILL_FREE set (via generic_forget_inode)
 */
static int
__writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
{
	wait_queue_head_t *wqh;

	if (!atomic_read(&inode->i_count))
		WARN_ON(!(inode->i_state & (I_WILL_FREE|I_FREEING)));
	else
		WARN_ON(inode->i_state & I_WILL_FREE);

	if ((wbc->sync_mode != WB_SYNC_ALL) && (inode->i_state & I_LOCK)) {
		struct address_space *mapping = inode->i_mapping;
		int ret;

		list_move(&inode->i_list, &inode->i_sb->s_dirty);

		/*
		 * Even if we don't actually write the inode itself here,
		 * we can at least start some of the data writeout..
		 */
		spin_unlock(&inode_lock);
		ret = do_writepages(mapping, wbc);
		spin_lock(&inode_lock);
		return ret;
	}

	/*
	 * It's a data-integrity sync.  We must wait.
	 */
	if (inode->i_state & I_LOCK) {
		DEFINE_WAIT_BIT(wq, &inode->i_state, __I_LOCK);

		wqh = bit_waitqueue(&inode->i_state, __I_LOCK);
		do {
			spin_unlock(&inode_lock);
			__wait_on_bit(wqh, &wq, inode_wait,
							TASK_UNINTERRUPTIBLE);
			spin_lock(&inode_lock);
		} while (inode->i_state & I_LOCK);
	}
	return __sync_single_inode(inode, wbc);
}

/*
 * Write out a superblock's list of dirty inodes.  A wait will be performed
 * upon no inodes, all inodes or the final one, depending upon sync_mode.
 *
 * If older_than_this is non-NULL, then only write out inodes which
 * had their first dirtying at a time earlier than *older_than_this.
 *
 * If we're a pdlfush thread, then implement pdflush collision avoidance
 * against the entire list.
 *
 * WB_SYNC_HOLD is a hack for sys_sync(): reattach the inode to sb->s_dirty so
 * that it can be located for waiting on in __writeback_single_inode().
 *
 * Called under inode_lock.
 *
 * If `bdi' is non-zero then we're being asked to writeback a specific queue.
 * This function assumes that the blockdev superblock's inodes are backed by
 * a variety of queues, so all inodes are searched.  For other superblocks,
 * assume that all inodes are backed by the same queue.
 *
 * FIXME: this linear search could get expensive with many fileystems.  But
 * how to fix?  We need to go from an address_space to all inodes which share
 * a queue with that address_space.  (Easy: have a global "dirty superblocks"
 * list).
 *
 * The inodes to be written are parked on sb->s_io.  They are moved back onto
 * sb->s_dirty as they are selected for writing.  This way, none can be missed
 * on the writer throttling path, and we get decent balancing between many
 * throttled threads: we don't want them all piling up on __wait_on_inode.
 */
static void
sync_sb_inodes(struct super_block *sb, struct writeback_control *wbc)
{
	const unsigned long start = jiffies;	/* livelock avoidance */

	if (!wbc->for_kupdate || list_empty(&sb->s_io))
		list_splice_init(&sb->s_dirty, &sb->s_io);

	while (!list_empty(&sb->s_io)) {
		struct inode *inode = list_entry(sb->s_io.prev,
						struct inode, i_list);
		struct address_space *mapping = inode->i_mapping;
		struct backing_dev_info *bdi = mapping->backing_dev_info;
		long pages_skipped;

		if (!bdi_cap_writeback_dirty(bdi)) {
			list_move(&inode->i_list, &sb->s_dirty);
			if (sb_is_blkdev_sb(sb)) {
				/*
				 * Dirty memory-backed blockdev: the ramdisk
				 * driver does this.  Skip just this inode
				 */
				continue;
			}
			/*
			 * Dirty memory-backed inode against a filesystem other
			 * than the kernel-internal bdev filesystem.  Skip the
			 * entire superblock.
			 */
			break;
		}

		if (wbc->nonblocking && bdi_write_congested(bdi)) {
			wbc->encountered_congestion = 1;
			if (!sb_is_blkdev_sb(sb))
				break;		/* Skip a congested fs */
			list_move(&inode->i_list, &sb->s_dirty);
			continue;		/* Skip a congested blockdev */
		}

		if (wbc->bdi && bdi != wbc->bdi) {
			if (!sb_is_blkdev_sb(sb))
				break;		/* fs has the wrong queue */
			list_move(&inode->i_list, &sb->s_dirty);
			continue;		/* blockdev has wrong queue */
		}

		/* Was this inode dirtied after sync_sb_inodes was called? */
		if (time_after(inode->dirtied_when, start))
			break;

		/* Was this inode dirtied too recently? */
		if (wbc->older_than_this && time_after(inode->dirtied_when,
						*wbc->older_than_this))
			break;

		/* Is another pdflush already flushing this queue? */
		if (current_is_pdflush() && !writeback_acquire(bdi))
			break;

		BUG_ON(inode->i_state & I_FREEING);
		__iget(inode);
		pages_skipped = wbc->pages_skipped;
		__writeback_single_inode(inode, wbc);
		if (wbc->sync_mode == WB_SYNC_HOLD) {
			inode->dirtied_when = jiffies;
			list_move(&inode->i_list, &sb->s_dirty);
		}
		if (current_is_pdflush())
			writeback_release(bdi);
		if (wbc->pages_skipped != pages_skipped) {
			/*
			 * writeback is not making progress due to locked
			 * buffers.  Skip this inode for now.
			 */
			list_move(&inode->i_list, &sb->s_dirty);
		}
		spin_unlock(&inode_lock);
		iput(inode);
		cond_resched();
		spin_lock(&inode_lock);
		if (wbc->nr_to_write <= 0)
			break;
	}
	return;		/* Leave any unwritten inodes on s_io */
}

/*
 * Start writeback of dirty pagecache data against all unlocked inodes.
 *
 * Note:
 * We don't need to grab a reference to superblock here. If it has non-empty
 * ->s_dirty it's hadn't been killed yet and kill_super() won't proceed
 * past sync_inodes_sb() until both the ->s_dirty and ->s_io lists are
 * empty. Since __sync_single_inode() regains inode_lock before it finally moves
 * inode from superblock lists we are OK.
 *
 * If `older_than_this' is non-zero then only flush inodes which have a
 * flushtime older than *older_than_this.
 *
 * If `bdi' is non-zero then we will scan the first inode against each
 * superblock until we find the matching ones.  One group will be the dirty
 * inodes against a filesystem.  Then when we hit the dummy blockdev superblock,
 * sync_sb_inodes will seekout the blockdev which matches `bdi'.  Maybe not
 * super-efficient but we're about to do a ton of I/O...
 */
void
writeback_inodes(struct writeback_control *wbc)
{
	struct super_block *sb;

	might_sleep();
	spin_lock(&sb_lock);
restart:
	sb = sb_entry(super_blocks.prev);
	for (; sb != sb_entry(&super_blocks); sb = sb_entry(sb->s_list.prev)) {
		if (!list_empty(&sb->s_dirty) || !list_empty(&sb->s_io)) {
			/* we're making our own get_super here */
			sb->s_count++;
			spin_unlock(&sb_lock);
			/*
			 * If we can't get the readlock, there's no sense in
			 * waiting around, most of the time the FS is going to
			 * be unmounted by the time it is released.
			 */
			if (down_read_trylock(&sb->s_umount)) {
				if (sb->s_root) {
					spin_lock(&inode_lock);
					sync_sb_inodes(sb, wbc);
					spin_unlock(&inode_lock);
				}
				up_read(&sb->s_umount);
			}
			spin_lock(&sb_lock);
			if (__put_super_and_need_restart(sb))
				goto restart;
		}
		if (wbc->nr_to_write <= 0)
			break;
	}