[PATCH] race condition fixing in sysfs_create_dir

From: Dennis Chen
Date: Fri Jul 26 2013 - 05:46:37 EST

The patch is trying its best to avoid creating a dir under a parent dir which is removing from
the system:

PATH0 (create a dir under 'PARENT/...') PATH1 (remove the 'PARENT/...')
sysfs_create_dir() { sysfs_remove_dir() {
... ...
if (kobj->parent) spin_lock(&sysfs_assoc_lock);
parent_sd = kobj->parent->sd; <----- kobj->sd = NULL;
else spin_unlock(&sysfs_assoc_lock);
parent_sd = &sysfs_root;

Suppose PATH1 enter the critical section first, then PATH0 begin to execute before kobj->sd
has been reset to NULL, possibly PATH0 will get a non-NULL parent_sd since lack of the
sysfs_assoc_lock protection in PATH0. In this case, PATH0 think it has a valid parent_sd which
can be freed by PATH1 in the followed, refer to the comments in the patch. Maybe we need
to figure out a perfect solution to solve the race condition, although the codes in question are
in slow path...

Signed-off-by: Dennis Chen <xschen@xxxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Tejun Heo <teheo@xxxxxxx>
Cc: Wang Cong <xiyou.wangcong@xxxxxxxxx>
fs/sysfs/dir.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/fs/sysfs/dir.c b/fs/sysfs/dir.c
index e068e74..114073d 100644
--- a/fs/sysfs/dir.c
+++ b/fs/sysfs/dir.c
@@ -746,13 +746,22 @@ int sysfs_create_dir(struct kobject * kobj)

+ spin_lock(&sysfs_assoc_lock);
if (kobj->parent)
parent_sd = kobj->parent->sd;
parent_sd = &sysfs_root;

- if (!parent_sd)
+ if (!parent_sd) {
+ spin_unlock(&sysfs_assoc_lock);
return -ENOENT;
+ }
+ spin_unlock(&sysfs_assoc_lock);
+ /* TODO: although the sysfs is in a slow path, but in the operation
+ * followed, we still have a window to let the sysfs_remove_dir to
+ * free the memory space pointered by parent_sd till we inc its ref
+ * count in __sysfs_add_one()
+ */

if (sysfs_ns_type(parent_sd))
ns = kobj->ktype->namespace(kobj);


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/