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Subject[PATCH v0] kernfs: Skip kernfs_put of parent from child node
Date
While adding kernfs node for child to the parent kernfs
node and when child node founds that parent kn count is
zero, then below comes like:

WARNING: fs/kernfs/dir.c:494 kernfs_get+0x64/0x88

This indicates that parent is in kernfs_put path/ or already
freed, and if the child node keeps continue to
make new kernfs node, then there is chance of
below race for parent node:

CPU0 CPU1
//Parent node //child node
kernfs_put
atomic_dec_and_test(&kn->count)
//count is 0, so continue
kernfs_new_node(child)
kernfs_get(parent);
//increment parent count to 1
//warning come as parent count is 0
/* link in */
kernfs_add_one(kn);
// this should fail as parent is
//in free path.
kernfs_put(child)
kmem_cache_free(parent)
kmem_cache_free(child)
kn = parent
atomic_dec_and_test(&kn->count))
//this is 0 now, so release will
continue for parent.
kmem_cache_free(parent)

To prevent this race, child simply has to decrement count of parent
kernfs node and keep continue the free path for itself.

Signed-off-by: Gaurav Kohli <gkohli@codeaurora.org>
Signed-off-by: Mukesh Ojha <mojha@codeaurora.org>

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index b84d635..d5a36e8 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -515,7 +515,6 @@ void kernfs_put(struct kernfs_node *kn)
if (!kn || !atomic_dec_and_test(&kn->count))
return;
root = kernfs_root(kn);
- repeat:
/*
* Moving/renaming is always done while holding reference.
* kn->parent won't change beneath us.
@@ -545,8 +544,8 @@ void kernfs_put(struct kernfs_node *kn)

kn = parent;
if (kn) {
- if (atomic_dec_and_test(&kn->count))
- goto repeat;
+ /* Parent may be on free path, so simply decrement the count */
+ atomic_dec_and_test(&kn->count);
} else {
/* just released the root kn, free @root too */
idr_destroy(&root->ino_idr);
--
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center,
Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project
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