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from multiple threads while holding a shared lock during a lookup operation. This could result in incorrect ENOENT failures which could then be permanently stored in the name cache. Specifically, the dirhash code optimizes the case that a single thread is walking a directory sequentially opening (or stat'ing) each file. It uses state in the dirhash structure to determine if a given lookup is using the optimization. If the optimization fails, it disables it and restarts the lookup. The problem arises when two threads both attempt the optimization and fail. The first thread will restart the loop, but the second thread will incorrectly think that it did not try the optimization and will only examine a subset of the directory entires in its hash chain. As a result, it may fail to find its directory entry and incorrectly fail with ENOENT. To make this safe for use with shared locks, simplify the state stored in the dirhash and move some of the state (the part that determines if the current thread is trying the optimization) into a local variable. One result is that we will now try the optimization more often. We still update the value under the shared lock, but it is a single atomic store similar to i_diroff that is stored in UFS directory i-nodes for the non-dirhash lookup. Reviewed by: kib MFC after: 1 week
134 lines
5.1 KiB
C
134 lines
5.1 KiB
C
/*-
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* Copyright (c) 2001 Ian Dowse. All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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*
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* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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* ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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* SUCH DAMAGE.
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*
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* $FreeBSD$
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*/
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#ifndef _UFS_UFS_DIRHASH_H_
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#define _UFS_UFS_DIRHASH_H_
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#include <sys/_lock.h>
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#include <sys/_sx.h>
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/*
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* For fast operations on large directories, we maintain a hash
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* that maps the file name to the offset of the directory entry within
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* the directory file.
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*
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* The hashing uses a dumb spillover to the next free slot on
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* collisions, so we must keep the utilisation low to avoid
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* long linear searches. Deleted entries that are not the last
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* in a chain must be marked DIRHASH_DEL.
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*
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* We also maintain information about free space in each block
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* to speed up creations.
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*/
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#define DIRHASH_EMPTY (-1) /* entry unused */
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#define DIRHASH_DEL (-2) /* deleted entry; may be part of chain */
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#define DIRALIGN 4
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#define DH_NFSTATS (DIRECTSIZ(MAXNAMLEN + 1) / DIRALIGN)
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/* max DIRALIGN words in a directory entry */
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/*
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* Dirhash uses a score mechanism to achieve a hybrid between a
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* least-recently-used and a least-often-used algorithm for entry
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* recycling. The score is incremented when a directory is used, and
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* decremented when the directory is a candidate for recycling. When
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* the score reaches zero, the hash is recycled. Hashes are linked
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* together on a TAILQ list, and hashes with higher scores filter
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* towards the tail (most recently used) end of the list.
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*
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* New hash entries are given an inital score of DH_SCOREINIT and are
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* placed at the most-recently-used end of the list. This helps a lot
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* in the worst-case case scenario where every directory access is
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* to a directory that is not hashed (i.e. the working set of hash
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* candidates is much larger than the configured memry limit). In this
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* case it limits the number of hash builds to 1/DH_SCOREINIT of the
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* number of accesses.
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*/
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#define DH_SCOREINIT 8 /* initial dh_score when dirhash built */
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#define DH_SCOREMAX 64 /* max dh_score value */
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/*
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* The main hash table has 2 levels. It is an array of pointers to
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* blocks of DH_NBLKOFF offsets.
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*/
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#define DH_BLKOFFSHIFT 8
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#define DH_NBLKOFF (1 << DH_BLKOFFSHIFT)
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#define DH_BLKOFFMASK (DH_NBLKOFF - 1)
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#define DH_ENTRY(dh, slot) \
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((dh)->dh_hash[(slot) >> DH_BLKOFFSHIFT][(slot) & DH_BLKOFFMASK])
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struct dirhash {
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struct sx dh_lock; /* protects all fields except list & score */
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int dh_refcount;
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doff_t **dh_hash; /* the hash array (2-level) */
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int dh_narrays; /* number of entries in dh_hash */
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int dh_hlen; /* total slots in the 2-level hash array */
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int dh_hused; /* entries in use */
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int dh_memreq; /* Memory used. */
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/* Free space statistics. XXX assumes DIRBLKSIZ is 512. */
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u_int8_t *dh_blkfree; /* free DIRALIGN words in each dir block */
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int dh_nblk; /* size of dh_blkfree array */
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int dh_dirblks; /* number of DIRBLKSIZ blocks in dir */
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int dh_firstfree[DH_NFSTATS + 1]; /* first blk with N words free */
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doff_t dh_seqoff; /* sequential access optimisation offset */
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int dh_score; /* access count for this dirhash */
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int dh_onlist; /* true if on the ufsdirhash_list chain */
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time_t dh_lastused; /* time the dirhash was last read or written*/
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/* Protected by ufsdirhash_mtx. */
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TAILQ_ENTRY(dirhash) dh_list; /* chain of all dirhashes */
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};
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/*
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* Dirhash functions.
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*/
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void ufsdirhash_init(void);
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void ufsdirhash_uninit(void);
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int ufsdirhash_build(struct inode *);
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doff_t ufsdirhash_findfree(struct inode *, int, int *);
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doff_t ufsdirhash_enduseful(struct inode *);
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int ufsdirhash_lookup(struct inode *, char *, int, doff_t *, struct buf **,
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doff_t *);
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void ufsdirhash_newblk(struct inode *, doff_t);
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void ufsdirhash_add(struct inode *, struct direct *, doff_t);
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void ufsdirhash_remove(struct inode *, struct direct *, doff_t);
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void ufsdirhash_move(struct inode *, struct direct *, doff_t, doff_t);
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void ufsdirhash_dirtrunc(struct inode *, doff_t);
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void ufsdirhash_free(struct inode *);
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void ufsdirhash_checkblock(struct inode *, char *, doff_t);
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#endif /* !_UFS_UFS_DIRHASH_H_ */
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