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Subject[PATCH 3/3] sysfs: fix race condition around sd->s_dentry, take#2
Allowing attribute and symlink dentries to be reclaimed means
sd->s_dentry can change dynamically. However, updates to the field
are unsynchronized leading to race conditions. This patch adds
sysfs_lock and use it to synchronize updates to sd->s_dentry.

Due to the locking around ->d_iput, the check in sysfs_drop_dentry()
is complex. sysfs_lock only protect sd->s_dentry pointer itself. The
validity of the dentry is protected by dcache_lock, so whether dentry
is alive or not can only be tested while holding both locks.

This is minimal backport of sysfs_drop_dentry() rewrite in devel
branch.

Signed-off-by: Tejun Heo <htejun@gmail.com>
---
fs/sysfs/dir.c | 22 ++++++++++++++++++++--
fs/sysfs/inode.c | 18 +++++++++++++++++-
fs/sysfs/sysfs.h | 1 +
3 files changed, 38 insertions(+), 3 deletions(-)
Index: work1/fs/sysfs/dir.c
===================================================================
--- work1.orig/fs/sysfs/dir.c
+++ work1/fs/sysfs/dir.c
@@ -13,14 +13,26 @@
#include "sysfs.h"

DECLARE_RWSEM(sysfs_rename_sem);
+spinlock_t sysfs_lock = SPIN_LOCK_UNLOCKED;

static void sysfs_d_iput(struct dentry * dentry, struct inode * inode)
{
struct sysfs_dirent * sd = dentry->d_fsdata;

if (sd) {
- BUG_ON(sd->s_dentry != dentry);
- sd->s_dentry = NULL;
+ /* sd->s_dentry is protected with sysfs_lock. This
+ * allows sysfs_drop_dentry() to dereference it.
+ */
+ spin_lock(&sysfs_lock);
+
+ /* The dentry might have been deleted or another
+ * lookup could have happened updating sd->s_dentry to
+ * point the new dentry. Ignore if it isn't pointing
+ * to this dentry.
+ */
+ if (sd->s_dentry == dentry)
+ sd->s_dentry = NULL;
+ spin_unlock(&sysfs_lock);
sysfs_put(sd);
}
iput(inode);
@@ -247,7 +259,10 @@ static int sysfs_attach_attr(struct sysf
}

dentry->d_fsdata = sysfs_get(sd);
+ /* protect sd->s_dentry against sysfs_d_iput */
+ spin_lock(&sysfs_lock);
sd->s_dentry = dentry;
+ spin_unlock(&sysfs_lock);
error = sysfs_create(dentry, (attr->mode & S_IALLUGO) | S_IFREG, init);
if (error) {
sysfs_put(sd);
@@ -269,7 +284,10 @@ static int sysfs_attach_link(struct sysf
int err = 0;

dentry->d_fsdata = sysfs_get(sd);
+ /* protect sd->s_dentry against sysfs_d_iput */
+ spin_lock(&sysfs_lock);
sd->s_dentry = dentry;
+ spin_unlock(&sysfs_lock);
err = sysfs_create(dentry, S_IFLNK|S_IRWXUGO, init_symlink);
if (!err) {
dentry->d_op = &sysfs_dentry_ops;
Index: work1/fs/sysfs/inode.c
===================================================================
--- work1.orig/fs/sysfs/inode.c
+++ work1/fs/sysfs/inode.c
@@ -246,9 +246,23 @@ static inline void orphan_all_buffers(st
*/
void sysfs_drop_dentry(struct sysfs_dirent * sd, struct dentry * parent)
{
- struct dentry * dentry = sd->s_dentry;
+ struct dentry *dentry = NULL;
struct inode *inode;

+ /* We're not holding a reference to ->s_dentry dentry but the
+ * field will stay valid as long as sysfs_lock is held.
+ */
+ spin_lock(&sysfs_lock);
+ spin_lock(&dcache_lock);
+
+ /* dget dentry if it's still alive */
+ if (sd->s_dentry && sd->s_dentry->d_inode)
+ dentry = dget_locked(sd->s_dentry);
+
+ spin_unlock(&dcache_lock);
+ spin_unlock(&sysfs_lock);
+
+ /* drop dentry */
if (dentry) {
spin_lock(&dcache_lock);
spin_lock(&dentry->d_lock);
@@ -268,6 +282,8 @@ void sysfs_drop_dentry(struct sysfs_dire
spin_unlock(&dentry->d_lock);
spin_unlock(&dcache_lock);
}
+
+ dput(dentry);
}
}

Index: work1/fs/sysfs/sysfs.h
===================================================================
--- work1.orig/fs/sysfs/sysfs.h
+++ work1/fs/sysfs/sysfs.h
@@ -33,6 +33,7 @@ extern const unsigned char * sysfs_get_n
extern void sysfs_drop_dentry(struct sysfs_dirent *sd, struct dentry *parent);
extern int sysfs_setattr(struct dentry *dentry, struct iattr *iattr);

+extern spinlock_t sysfs_lock;
extern struct rw_semaphore sysfs_rename_sem;
extern struct super_block * sysfs_sb;
extern const struct file_operations sysfs_dir_operations;
-
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