rehash process protected with d_seq and d_lock locks, but VFS have access to the
d_hashed field without any locks sometimes. It produce errors with get cwd
operations or fsnotify may report an unlink event sometimes. d_seq lock isn’t
used to protect due possibility to sleep with holding locks, and d_lock is too
hot to check a simple state, lets return a DCACHE_DENTRY_UNHASHED flag to
ability to check a unhashed state without locks held.
#include <pthread.h>
#include <sys/stat.h>
#include <stdlib.h>
#include <assert.h>
#include <unistd.h>
#include <stdio.h>
#include <errno.h>
void *thread_main(void *unused)
{
int rc;
for (;;) {
rc = rename("/tmp/t-a", "/tmp/t-b");
assert(rc == 0);
rc = rename("/tmp/t-b", "/tmp/t-a");
assert(rc == 0);
}
return NULL;
}
int main(void) {
int rc, i;
pthread_t ptt;
rmdir("/tmp/t-a");
rmdir("/tmp/t-b");
rc = mkdir("/tmp/t-a", 0666);
assert(rc == 0);
rc = chdir("/tmp/t-a");
assert(rc == 0);
rc = pthread_create(&ptt, NULL, thread_main, NULL);
assert(rc == 0);
for (i=0; ;i++) {
char buf[100], *b;
b = getcwd(buf, sizeof(buf));
if (b == NULL) {
printf("getcwd failed on iter %d with %d\n", i, errno);
break;
}
}
return 0;
}
Reported-by: Andrew Perepechko <anserper@xxxxxxxxx>
Signed-off-by: Alexey Lyashkov <alexey.lyashkov@xxxxxxxxx>
---
fs/configfs/inode.c | 2 +-
fs/dcache.c | 24 ++++++++++++++++--------
fs/nfs/dir.c | 2 +-
include/linux/dcache.h | 6 ++++--
4 files changed, 22 insertions(+), 12 deletions(-)
diff --git a/fs/configfs/inode.c b/fs/configfs/inode.c
index ad718e5..9f531f7 100644
--- a/fs/configfs/inode.c
+++ b/fs/configfs/inode.c
@@ -248,7 +248,7 @@ void configfs_drop_dentry(struct configfs_dirent * sd, struct dentry * parent)
spin_lock(&dentry->d_lock);
if (simple_positive(dentry)) {
dget_dlock(dentry);
- __d_drop(dentry);
+ _d_drop(dentry);
spin_unlock(&dentry->d_lock);
simple_unlink(d_inode(parent), dentry);
} else
diff --git a/fs/dcache.c b/fs/dcache.c
index 5c7cc95..0863841 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -438,7 +438,7 @@ static void dentry_lru_add(struct dentry *dentry)
*/
void __d_drop(struct dentry *dentry)
{
- if (!d_unhashed(dentry)) {
+ if (!hlist_bl_unhashed(&dentry->d_hash)) {
struct hlist_bl_head *b;
/*
* Hashed dentries are normally on the dentry hashtable,
@@ -458,12 +458,18 @@ void __d_drop(struct dentry *dentry)
write_seqcount_invalidate(&dentry->d_seq);
}
}
-EXPORT_SYMBOL(__d_drop);
+
+void _d_drop(struct dentry *dentry)
+{
+ dentry->d_flags |= DCACHE_DENTRY_UNHASHED;
+ __d_drop(dentry);
+}
+EXPORT_SYMBOL(_d_drop);
void d_drop(struct dentry *dentry)
{
spin_lock(&dentry->d_lock);
- __d_drop(dentry);
+ _d_drop(dentry);
spin_unlock(&dentry->d_lock);
}
EXPORT_SYMBOL(d_drop);
@@ -530,7 +536,7 @@ static void __dentry_kill(struct dentry *dentry)
d_lru_del(dentry);
}
/* if it was on the hash then remove it */
- __d_drop(dentry);
+ _d_drop(dentry);
dentry_unlist(dentry, parent);
if (parent)
spin_unlock(&parent->d_lock);
@@ -1486,7 +1492,7 @@ static void check_and_drop(void *_data)
struct detach_data *data = _data;
if (!data->mountpoint && !data->select.found)
- __d_drop(data->select.start);
+ _d_drop(data->select.start);
}
/**
@@ -1601,7 +1607,7 @@ struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
dentry->d_name.name = dname;
dentry->d_lockref.count = 1;
- dentry->d_flags = 0;
+ dentry->d_flags = DCACHE_DENTRY_UNHASHED;
spin_lock_init(&dentry->d_lock);
seqcount_init(&dentry->d_seq);
dentry->d_inode = NULL;
@@ -1926,6 +1932,7 @@ static struct dentry *__d_obtain_alias(struct inode *inode, int disconnected)
spin_lock(&tmp->d_lock);
__d_set_inode_and_type(tmp, inode, add_flags);
hlist_add_head(&tmp->d_u.d_alias, &inode->i_dentry);
+ tmp->d_flags &= ~DCACHE_DENTRY_UNHASHED;
hlist_bl_lock(&tmp->d_sb->s_anon);
hlist_bl_add_head(&tmp->d_hash, &tmp->d_sb->s_anon);
hlist_bl_unlock(&tmp->d_sb->s_anon);
@@ -2331,7 +2338,7 @@ void d_delete(struct dentry * dentry)
}
if (!d_unhashed(dentry))
- __d_drop(dentry);
+ _d_drop(dentry);
spin_unlock(&dentry->d_lock);
@@ -2342,7 +2349,8 @@ EXPORT_SYMBOL(d_delete);
static void __d_rehash(struct dentry *entry)
{
struct hlist_bl_head *b = d_hash(entry->d_name.hash);
- BUG_ON(!d_unhashed(entry));
+ BUG_ON(!hlist_bl_unhashed(&entry->d_hash));
+ entry->d_flags &= ~DCACHE_DENTRY_UNHASHED;
hlist_bl_lock(b);
hlist_bl_add_head_rcu(&entry->d_hash, b);
hlist_bl_unlock(b);
diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index 5f1af4c..a1911bb 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -1891,7 +1891,7 @@ int nfs_unlink(struct inode *dir, struct dentry *dentry)
goto out;
}
if (!d_unhashed(dentry)) {
- __d_drop(dentry);
+ _d_drop(dentry);
need_rehash = 1;
}
spin_unlock(&dentry->d_lock);
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 5beed7b..6b0b3d7 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -213,6 +213,7 @@ struct dentry_operations {
#define DCACHE_PAR_LOOKUP 0x10000000 /* being looked up (with parent locked shared) */
#define DCACHE_DENTRY_CURSOR 0x20000000
+#define DCACHE_DENTRY_UNHASHED 0x40000000 /* dentry unhashed from tree with unlink */
extern seqlock_t rename_lock;
/*
@@ -221,7 +222,7 @@ extern seqlock_t rename_lock;
extern void d_instantiate(struct dentry *, struct inode *);
extern struct dentry * d_instantiate_unique(struct dentry *, struct inode *);
extern int d_instantiate_no_diralias(struct dentry *, struct inode *);
-extern void __d_drop(struct dentry *dentry);
+extern void _d_drop(struct dentry *dentry);
extern void d_drop(struct dentry *dentry);
extern void d_delete(struct dentry *);
extern void d_set_d_op(struct dentry *dentry, const struct dentry_operations *op);
@@ -326,11 +327,12 @@ extern struct dentry *dget_parent(struct dentry *dentry);
* @dentry: entry to check
*
* Returns true if the dentry passed is not currently hashed.
+ * hlist_bl_unhashed can't be used as race with d_move().
*/
static inline int d_unhashed(const struct dentry *dentry)
{
- return hlist_bl_unhashed(&dentry->d_hash);
+ return dentry->d_flags & DCACHE_DENTRY_UNHASHED;
}
static inline int d_unlinked(const struct dentry *dentry)
--
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