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SubjectRe: deferring __fput()
On 06/23, Al Viro wrote:
>
> You know what... Let's dump that "reverse the list" thing completely.

Yes, it was never really needed for fifo.

> What we ought to
> do instead of that is honestly keeping both the head of the (single-linked) list and
> pointer to pointer to its last element. Sure, that'll eat one more word in task_struct.
> And it doesn't matter, since we'll be able to kill something else in there - namely,
> ->scm_work_list.

Still it is better to not add the second pointer, task->task_works can
point to the last work, and last_work->next points to the first one.

> 1) kill task_work->data; the only user that cares will allocate task_work + struct cred * and
> use container_of() to get to it.

OK. See the patch below. I need to cleanup it somehow (and test of course),
cred.h needs task_work.h but task_work.h needs task_struct.

And, David, can you suggest the good name for cred->xxx?

> 2) replace current->task_works with struct task_work *task_works, **last_work and replace
> hlist_node in task_work with struct task_work *next;

Yes, but see above.

> 3) at that point task_work is equal in size (and layout, BTW) to rcu_head.

Yep.

> [... snip ...]

I'll try to understand fput/scm issues later ;)

Oleg.


diff --git a/include/linux/cred.h b/include/linux/cred.h
index ebbed2c..d364255 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -18,6 +18,7 @@
#include <linux/selinux.h>
#include <linux/atomic.h>
#include <linux/uidgid.h>
+#include <linux/task_work.h>

struct user_struct;
struct cred;
@@ -149,7 +150,10 @@ struct cred {
struct user_struct *user; /* real user ID subscription */
struct user_namespace *user_ns; /* user_ns the caps and keyrings are relative to. */
struct group_info *group_info; /* supplementary groups for euid/fsgid */
- struct rcu_head rcu; /* RCU deletion hook */
+ union {
+ struct rcu_head rcu; /* RCU deletion hook */
+ struct task_work xxx; /* keyctl_session_to_parent() */
+ };
};

extern void __put_cred(struct cred *);
diff --git a/include/linux/task_work.h b/include/linux/task_work.h
index 294d5d5..f834e38 100644
--- a/include/linux/task_work.h
+++ b/include/linux/task_work.h
@@ -1,9 +1,6 @@
#ifndef _LINUX_TASK_WORK_H
#define _LINUX_TASK_WORK_H

-#include <linux/list.h>
-#include <linux/sched.h>
-
struct task_work;
typedef void (*task_work_func_t)(struct task_work *);

@@ -24,10 +21,14 @@ int task_work_add(struct task_struct *task, struct task_work *twork, bool);
struct task_work *task_work_cancel(struct task_struct *, task_work_func_t);
void task_work_run(void);

-static inline void exit_task_work(struct task_struct *task)
-{
- if (unlikely(!hlist_empty(&task->task_works)))
- task_work_run();
-}
+#define exit_task_work(__task) \
+ do { \
+ struct task_struct *task = (__task); \
+ if (unlikely(!hlist_empty(&task->task_works))) \
+ task_work_run(); \
+ } while (0)
+
+#include <linux/list.h>
+#include <linux/sched.h>

#endif /* _LINUX_TASK_WORK_H */
diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index 0f5b3f0..cc428c9 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -1456,8 +1456,8 @@ long keyctl_session_to_parent(void)
{
struct task_struct *me, *parent;
const struct cred *mycred, *pcred;
- struct task_work *newwork, *oldwork;
key_ref_t keyring_r;
+ struct task_work *oldwork;
struct cred *cred;
int ret;

@@ -1465,20 +1465,16 @@ long keyctl_session_to_parent(void)
if (IS_ERR(keyring_r))
return PTR_ERR(keyring_r);

- ret = -ENOMEM;
- newwork = kmalloc(sizeof(struct task_work), GFP_KERNEL);
- if (!newwork)
- goto error_keyring;
-
/* our parent is going to need a new cred struct, a new tgcred struct
* and new security data, so we allocate them here to prevent ENOMEM in
* our parent */
+ ret = -ENOMEM;
cred = cred_alloc_blank();
if (!cred)
- goto error_newwork;
+ goto error_keyring;

cred->tgcred->session_keyring = key_ref_to_ptr(keyring_r);
- init_task_work(newwork, key_change_session_keyring, cred);
+ init_task_work(&cred->xxx, key_change_session_keyring, NULL);

me = current;
rcu_read_lock();
@@ -1527,24 +1523,19 @@ long keyctl_session_to_parent(void)

/* the replacement session keyring is applied just prior to userspace
* restarting */
- ret = task_work_add(parent, newwork, true);
+ ret = task_work_add(parent, &cred->xxx, true);
if (!ret)
- newwork = NULL;
+ cred = NULL;
unlock:
write_unlock_irq(&tasklist_lock);
rcu_read_unlock();
- if (oldwork) {
- put_cred(oldwork->data);
- kfree(oldwork);
- }
- if (newwork) {
- put_cred(newwork->data);
- kfree(newwork);
- }
+
+ if (oldwork)
+ put_cred(container_of(oldwork, struct cred, xxx));
+ if (cred)
+ put_cred(cred);
return ret;

-error_newwork:
- kfree(newwork);
error_keyring:
key_ref_put(keyring_r);
return ret;
diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
index 4ad54ee..5596caf 100644
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -837,9 +837,8 @@ error:
void key_change_session_keyring(struct task_work *twork)
{
const struct cred *old = current_cred();
- struct cred *new = twork->data;
+ struct cred *new = container_of(twork, struct cred, xxx);

- kfree(twork);
if (unlikely(current->flags & PF_EXITING)) {
put_cred(new);
return;


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