diff options
author | Al Viro <viro@zeniv.linux.org.uk> | 2016-06-10 15:32:47 (GMT) |
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committer | Al Viro <viro@zeniv.linux.org.uk> | 2016-06-10 15:32:47 (GMT) |
commit | ba65dc5ef16f82fba77869cecf7a7d515f61446b (patch) | |
tree | 58368a848f6b396be2ffcc2a43a4bee44fb354ef | |
parent | 1607f09c226d1378439c411baaaa020042750338 (diff) | |
download | linux-ba65dc5ef16f82fba77869cecf7a7d515f61446b.tar.xz |
much milder d_walk() race
d_walk() relies upon the tree not getting rearranged under it without
rename_lock being touched. And we do grab rename_lock around the
places that change the tree topology. Unfortunately, branch reordering
is just as bad from d_walk() POV and we have two places that do it
without touching rename_lock - one in handling of cursors (for ramfs-style
directories) and another in autofs. autofs one is a separate story; this
commit deals with the cursors.
* mark cursor dentries explicitly at allocation time
* make __dentry_kill() leave ->d_child.next pointing to the next
non-cursor sibling, making sure that it won't be moved around unnoticed
before the parent is relocked on ascend-to-parent path in d_walk().
* make d_walk() skip cursors explicitly; strictly speaking it's
not necessary (all callbacks we pass to d_walk() are no-ops on cursors),
but it makes analysis easier.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
-rw-r--r-- | fs/dcache.c | 58 | ||||
-rw-r--r-- | fs/internal.h | 1 | ||||
-rw-r--r-- | fs/libfs.c | 4 | ||||
-rw-r--r-- | include/linux/dcache.h | 1 |
4 files changed, 55 insertions, 9 deletions
diff --git a/fs/dcache.c b/fs/dcache.c index 817c243..b7eddfd 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -507,6 +507,44 @@ void d_drop(struct dentry *dentry) } EXPORT_SYMBOL(d_drop); +static inline void dentry_unlist(struct dentry *dentry, struct dentry *parent) +{ + struct dentry *next; + /* + * Inform d_walk() and shrink_dentry_list() that we are no longer + * attached to the dentry tree + */ + dentry->d_flags |= DCACHE_DENTRY_KILLED; + if (unlikely(list_empty(&dentry->d_child))) + return; + __list_del_entry(&dentry->d_child); + /* + * Cursors can move around the list of children. While we'd been + * a normal list member, it didn't matter - ->d_child.next would've + * been updated. However, from now on it won't be and for the + * things like d_walk() it might end up with a nasty surprise. + * Normally d_walk() doesn't care about cursors moving around - + * ->d_lock on parent prevents that and since a cursor has no children + * of its own, we get through it without ever unlocking the parent. + * There is one exception, though - if we ascend from a child that + * gets killed as soon as we unlock it, the next sibling is found + * using the value left in its ->d_child.next. And if _that_ + * pointed to a cursor, and cursor got moved (e.g. by lseek()) + * before d_walk() regains parent->d_lock, we'll end up skipping + * everything the cursor had been moved past. + * + * Solution: make sure that the pointer left behind in ->d_child.next + * points to something that won't be moving around. I.e. skip the + * cursors. + */ + while (dentry->d_child.next != &parent->d_subdirs) { + next = list_entry(dentry->d_child.next, struct dentry, d_child); + if (likely(!(next->d_flags & DCACHE_DENTRY_CURSOR))) + break; + dentry->d_child.next = next->d_child.next; + } +} + static void __dentry_kill(struct dentry *dentry) { struct dentry *parent = NULL; @@ -532,12 +570,7 @@ static void __dentry_kill(struct dentry *dentry) } /* if it was on the hash then remove it */ __d_drop(dentry); - __list_del_entry(&dentry->d_child); - /* - * Inform d_walk() that we are no longer attached to the - * dentry tree - */ - dentry->d_flags |= DCACHE_DENTRY_KILLED; + dentry_unlist(dentry, parent); if (parent) spin_unlock(&parent->d_lock); dentry_iput(dentry); @@ -1203,6 +1236,9 @@ resume: struct dentry *dentry = list_entry(tmp, struct dentry, d_child); next = tmp->next; + if (unlikely(dentry->d_flags & DCACHE_DENTRY_CURSOR)) + continue; + spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED); ret = enter(data, dentry); @@ -1651,6 +1687,16 @@ struct dentry *d_alloc(struct dentry * parent, const struct qstr *name) } EXPORT_SYMBOL(d_alloc); +struct dentry *d_alloc_cursor(struct dentry * parent) +{ + struct dentry *dentry = __d_alloc(parent->d_sb, NULL); + if (dentry) { + dentry->d_flags |= DCACHE_RCUACCESS | DCACHE_DENTRY_CURSOR; + dentry->d_parent = dget(parent); + } + return dentry; +} + /** * d_alloc_pseudo - allocate a dentry (for lookup-less filesystems) * @sb: the superblock diff --git a/fs/internal.h b/fs/internal.h index b71deee..f57ced5 100644 --- a/fs/internal.h +++ b/fs/internal.h @@ -130,6 +130,7 @@ extern int invalidate_inodes(struct super_block *, bool); extern struct dentry *__d_alloc(struct super_block *, const struct qstr *); extern int d_set_mounted(struct dentry *dentry); extern long prune_dcache_sb(struct super_block *sb, struct shrink_control *sc); +extern struct dentry *d_alloc_cursor(struct dentry *); /* * read_write.c @@ -71,9 +71,7 @@ EXPORT_SYMBOL(simple_lookup); int dcache_dir_open(struct inode *inode, struct file *file) { - static struct qstr cursor_name = QSTR_INIT(".", 1); - - file->private_data = d_alloc(file->f_path.dentry, &cursor_name); + file->private_data = d_alloc_cursor(file->f_path.dentry); return file->private_data ? 0 : -ENOMEM; } diff --git a/include/linux/dcache.h b/include/linux/dcache.h index 484c879..bcd0c64 100644 --- a/include/linux/dcache.h +++ b/include/linux/dcache.h @@ -212,6 +212,7 @@ struct dentry_operations { #define DCACHE_OP_REAL 0x08000000 #define DCACHE_PAR_LOOKUP 0x10000000 /* being looked up (with parent locked shared) */ +#define DCACHE_DENTRY_CURSOR 0x20000000 extern seqlock_t rename_lock; |