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 <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Wang Cong <[email protected]>
---
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)
BUG_ON(!kobj);
+ spin_lock(&sysfs_assoc_lock);
if (kobj->parent)
parent_sd = kobj->parent->sd;
else
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);
--
1.7.10.4
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