diff options
| -rw-r--r-- | fs/dcache.c | 196 |
1 files changed, 133 insertions, 63 deletions
diff --git a/fs/dcache.c b/fs/dcache.c index 761e31bacbc2..38b1b0989a16 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -88,6 +88,44 @@ EXPORT_SYMBOL(rename_lock); static struct kmem_cache *dentry_cache __read_mostly; +/** + * read_seqbegin_or_lock - begin a sequence number check or locking block + * lock: sequence lock + * seq : sequence number to be checked + * + * First try it once optimistically without taking the lock. If that fails, + * take the lock. The sequence number is also used as a marker for deciding + * whether to be a reader (even) or writer (odd). + * N.B. seq must be initialized to an even number to begin with. + */ +static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq) +{ + if (!(*seq & 1)) { /* Even */ + *seq = read_seqbegin(lock); + rcu_read_lock(); + } else /* Odd */ + write_seqlock(lock); +} + +/** + * read_seqretry_or_unlock - end a seqretry or lock block & return retry status + * lock : sequence lock + * seq : sequence number + * Return: 1 to retry operation again, 0 to continue + */ +static inline int read_seqretry_or_unlock(seqlock_t *lock, int *seq) +{ + if (!(*seq & 1)) { /* Even */ + rcu_read_unlock(); + if (read_seqretry(lock, *seq)) { + (*seq)++; /* Take writer lock */ + return 1; + } + } else /* Odd */ + write_sequnlock(lock); + return 0; +} + /* * This is the single most critical data structure when it comes * to the dcache: the hashtable for lookups. Somebody should try @@ -2644,9 +2682,39 @@ static int prepend(char **buffer, int *buflen, const char *str, int namelen) return 0; } +/** + * prepend_name - prepend a pathname in front of current buffer pointer + * buffer: buffer pointer + * buflen: allocated length of the buffer + * name: name string and length qstr structure + * + * With RCU path tracing, it may race with d_move(). Use ACCESS_ONCE() to + * make sure that either the old or the new name pointer and length are + * fetched. However, there may be mismatch between length and pointer. + * The length cannot be trusted, we need to copy it byte-by-byte until + * the length is reached or a null byte is found. It also prepends "/" at + * the beginning of the name. The sequence number check at the caller will + * retry it again when a d_move() does happen. So any garbage in the buffer + * due to mismatched pointer and length will be discarded. + */ static int prepend_name(char **buffer, int *buflen, struct qstr *name) { - return prepend(buffer, buflen, name->name, name->len); + const char *dname = ACCESS_ONCE(name->name); + u32 dlen = ACCESS_ONCE(name->len); + char *p; + + if (*buflen < dlen + 1) + return -ENAMETOOLONG; + *buflen -= dlen + 1; + p = *buffer -= dlen + 1; + *p++ = '/'; + while (dlen--) { + char c = *dname++; + if (!c) + break; + *p++ = c; + } + return 0; } /** @@ -2656,7 +2724,14 @@ static int prepend_name(char **buffer, int *buflen, struct qstr *name) * @buffer: pointer to the end of the buffer * @buflen: pointer to buffer length * - * Caller holds the rename_lock. + * The function tries to write out the pathname without taking any lock other + * than the RCU read lock to make sure that dentries won't go away. It only + * checks the sequence number of the global rename_lock as any change in the + * dentry's d_seq will be preceded by changes in the rename_lock sequence + * number. If the sequence number had been change, it will restart the whole + * pathname back-tracing sequence again. It performs a total of 3 trials of + * lockless back-tracing sequences before falling back to take the + * rename_lock. */ static int prepend_path(const struct path *path, const struct path *root, @@ -2665,54 +2740,60 @@ static int prepend_path(const struct path *path, struct dentry *dentry = path->dentry; struct vfsmount *vfsmnt = path->mnt; struct mount *mnt = real_mount(vfsmnt); - bool slash = false; int error = 0; + unsigned seq = 0; + char *bptr; + int blen; +restart: + bptr = *buffer; + blen = *buflen; + read_seqbegin_or_lock(&rename_lock, &seq); while (dentry != root->dentry || vfsmnt != root->mnt) { struct dentry * parent; if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) { /* Global root? */ - if (!mnt_has_parent(mnt)) - goto global_root; - dentry = mnt->mnt_mountpoint; - mnt = mnt->mnt_parent; - vfsmnt = &mnt->mnt; - continue; + if (mnt_has_parent(mnt)) { + dentry = mnt->mnt_mountpoint; + mnt = mnt->mnt_parent; + vfsmnt = &mnt->mnt; + continue; + } + /* + * Filesystems needing to implement special "root names" + * should do so with ->d_dname() + */ + if (IS_ROOT(dentry) && + (dentry->d_name.len != 1 || + dentry->d_name.name[0] != '/')) { + WARN(1, "Root dentry has weird name <%.*s>\n", + (int) dentry->d_name.len, + dentry->d_name.name); + } + if (!error) + error = is_mounted(vfsmnt) ? 1 : 2; + break; } parent = dentry->d_parent; prefetch(parent); - spin_lock(&dentry->d_lock); - error = prepend_name(buffer, buflen, &dentry->d_name); - spin_unlock(&dentry->d_lock); - if (!error) - error = prepend(buffer, buflen, "/", 1); + error = prepend_name(&bptr, &blen, &dentry->d_name); if (error) break; - slash = true; dentry = parent; } + if (read_seqretry_or_unlock(&rename_lock, &seq)) + goto restart; - if (!error && !slash) - error = prepend(buffer, buflen, "/", 1); - - return error; - -global_root: - /* - * Filesystems needing to implement special "root names" - * should do so with ->d_dname() - */ - if (IS_ROOT(dentry) && - (dentry->d_name.len != 1 || dentry->d_name.name[0] != '/')) { - WARN(1, "Root dentry has weird name <%.*s>\n", - (int) dentry->d_name.len, dentry->d_name.name); - } - if (!slash) - error = prepend(buffer, buflen, "/", 1); - if (!error) - error = is_mounted(vfsmnt) ? 1 : 2; + if (error >= 0 && bptr == *buffer) { + if (--blen < 0) + error = -ENAMETOOLONG; + else + *--bptr = '/'; + } + *buffer = bptr; + *buflen = blen; return error; } @@ -2741,9 +2822,7 @@ char *__d_path(const struct path *path, prepend(&res, &buflen, "\0", 1); br_read_lock(&vfsmount_lock); - write_seqlock(&rename_lock); error = prepend_path(path, root, &res, &buflen); - write_sequnlock(&rename_lock); br_read_unlock(&vfsmount_lock); if (error < 0) @@ -2762,9 +2841,7 @@ char *d_absolute_path(const struct path *path, prepend(&res, &buflen, "\0", 1); br_read_lock(&vfsmount_lock); - write_seqlock(&rename_lock); error = prepend_path(path, &root, &res, &buflen); - write_sequnlock(&rename_lock); br_read_unlock(&vfsmount_lock); if (error > 1) @@ -2830,9 +2907,7 @@ char *d_path(const struct path *path, char *buf, int buflen) get_fs_root(current->fs, &root); br_read_lock(&vfsmount_lock); - write_seqlock(&rename_lock); error = path_with_deleted(path, &root, &res, &buflen); - write_sequnlock(&rename_lock); br_read_unlock(&vfsmount_lock); if (error < 0) res = ERR_PTR(error); @@ -2867,10 +2942,10 @@ char *simple_dname(struct dentry *dentry, char *buffer, int buflen) char *end = buffer + buflen; /* these dentries are never renamed, so d_lock is not needed */ if (prepend(&end, &buflen, " (deleted)", 11) || - prepend_name(&end, &buflen, &dentry->d_name) || + prepend(&end, &buflen, dentry->d_name.name, dentry->d_name.len) || prepend(&end, &buflen, "/", 1)) end = ERR_PTR(-ENAMETOOLONG); - return end; + return end; } /* @@ -2878,30 +2953,36 @@ char *simple_dname(struct dentry *dentry, char *buffer, int buflen) */ static char *__dentry_path(struct dentry *dentry, char *buf, int buflen) { - char *end = buf + buflen; - char *retval; + char *end, *retval; + int len, seq = 0; + int error = 0; - prepend(&end, &buflen, "\0", 1); +restart: + end = buf + buflen; + len = buflen; + prepend(&end, &len, "\0", 1); if (buflen < 1) goto Elong; /* Get '/' right */ retval = end-1; *retval = '/'; - + read_seqbegin_or_lock(&rename_lock, &seq); while (!IS_ROOT(dentry)) { struct dentry *parent = dentry->d_parent; int error; prefetch(parent); - spin_lock(&dentry->d_lock); - error = prepend_name(&end, &buflen, &dentry->d_name); - spin_unlock(&dentry->d_lock); - if (error != 0 || prepend(&end, &buflen, "/", 1) != 0) - goto Elong; + error = prepend_name(&end, &len, &dentry->d_name); + if (error) + break; retval = end; dentry = parent; } + if (read_seqretry_or_unlock(&rename_lock, &seq)) + goto restart; + if (error) + goto Elong; return retval; Elong: return ERR_PTR(-ENAMETOOLONG); @@ -2909,13 +2990,7 @@ Elong: char *dentry_path_raw(struct dentry *dentry, char *buf, int buflen) { - char *retval; - - write_seqlock(&rename_lock); - retval = __dentry_path(dentry, buf, buflen); - write_sequnlock(&rename_lock); - - return retval; + return __dentry_path(dentry, buf, buflen); } EXPORT_SYMBOL(dentry_path_raw); @@ -2924,7 +2999,6 @@ char *dentry_path(struct dentry *dentry, char *buf, int buflen) char *p = NULL; char *retval; - write_seqlock(&rename_lock); if (d_unlinked(dentry)) { p = buf + buflen; if (prepend(&p, &buflen, "//deleted", 10) != 0) @@ -2932,7 +3006,6 @@ char *dentry_path(struct dentry *dentry, char *buf, int buflen) buflen++; } retval = __dentry_path(dentry, buf, buflen); - write_sequnlock(&rename_lock); if (!IS_ERR(retval) && p) *p = '/'; /* restore '/' overriden with '\0' */ return retval; @@ -2971,7 +3044,6 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size) error = -ENOENT; br_read_lock(&vfsmount_lock); - write_seqlock(&rename_lock); if (!d_unlinked(pwd.dentry)) { unsigned long len; char *cwd = page + PAGE_SIZE; @@ -2979,7 +3051,6 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size) prepend(&cwd, &buflen, "\0", 1); error = prepend_path(&pwd, &root, &cwd, &buflen); - write_sequnlock(&rename_lock); br_read_unlock(&vfsmount_lock); if (error < 0) @@ -3000,7 +3071,6 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size) error = -EFAULT; } } else { - write_sequnlock(&rename_lock); br_read_unlock(&vfsmount_lock); } |