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Date: Mon, 20 Aug 2012 13:44:41 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Dave Jones <davej@...hat.com>,
Linux Kernel <linux-kernel@...r.kernel.org>,
Thomas Gleixner <tglx@...utronix.de>,
rostedt <rostedt@...dmis.org>, dhowells <dhowells@...hat.com>,
Al Viro <viro@...iv.linux.org.uk>
Subject: Re: lockdep trace from posix timers
On Mon, 2012-08-20 at 09:15 +0200, Peter Zijlstra wrote:
> On Fri, 2012-08-17 at 17:14 +0200, Oleg Nesterov wrote:
> > I still think that task_work_add() should synhronize with exit_task_work()
> > itself and fail if necessary. But I wasn't able to convince Al ;)
>
> I'm not at all sure how that relates to needing task_lock() in the
> keyctl stuff.
>
> Also, can't task_work use llist stuff? That would also avoid using
> ->pi_lock.
How about something like the below?
---
include/linux/task_work.h | 7 +--
kernel/exit.c | 2 +-
kernel/task_work.c | 120 ++++++++++++++++++++++++----------------------
3 files changed, 65 insertions(+), 64 deletions(-)
diff --git a/include/linux/task_work.h b/include/linux/task_work.h
index fb46b03..f365416 100644
--- a/include/linux/task_work.h
+++ b/include/linux/task_work.h
@@ -15,11 +15,6 @@ init_task_work(struct callback_head *twork, task_work_func_t func)
int task_work_add(struct task_struct *task, struct callback_head *twork, bool);
struct callback_head *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(task->task_works))
- task_work_run();
-}
+void task_work_exit(void);
#endif /* _LINUX_TASK_WORK_H */
diff --git a/kernel/exit.c b/kernel/exit.c
index f65345f..92aa94b 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -992,7 +992,7 @@ void do_exit(long code)
exit_shm(tsk);
exit_files(tsk);
exit_fs(tsk);
- exit_task_work(tsk);
+ task_work_exit();
check_stack_usage();
exit_thread();
diff --git a/kernel/task_work.c b/kernel/task_work.c
index 91d4e17..e5eac14 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -2,79 +2,85 @@
#include <linux/task_work.h>
#include <linux/tracehook.h>
+static void task_work_nop(struct callback_head *work)
+{
+}
+
+static struct callback_head dead = {
+ .next = NULL,
+ .func = task_work_nop,
+};
+
int
-task_work_add(struct task_struct *task, struct callback_head *twork, bool notify)
+task_work_add(struct task_struct *task, struct callback_head *work, bool notify)
{
- struct callback_head *last, *first;
- unsigned long flags;
-
- /*
- * Not inserting the new work if the task has already passed
- * exit_task_work() is the responisbility of callers.
- */
- raw_spin_lock_irqsave(&task->pi_lock, flags);
- last = task->task_works;
- first = last ? last->next : twork;
- twork->next = first;
- if (last)
- last->next = twork;
- task->task_works = twork;
- raw_spin_unlock_irqrestore(&task->pi_lock, flags);
+ struct callback_head **head = &task->task_works;
+ struct callback_head *entry, *old_entry;
+
+ entry = &head;
+ for (;;) {
+ if (entry == &dead)
+ return -ESRCH;
+
+ old_entry = entry;
+ work->next = entry;
+ entry = cmpxchg(head, old_entry, work);
+ if (entry == old_entry)
+ break;
+ }
/* test_and_set_bit() implies mb(), see tracehook_notify_resume(). */
if (notify)
set_notify_resume(task);
+
return 0;
}
struct callback_head *
task_work_cancel(struct task_struct *task, task_work_func_t func)
{
- unsigned long flags;
- struct callback_head *last, *res = NULL;
-
- raw_spin_lock_irqsave(&task->pi_lock, flags);
- last = task->task_works;
- if (last) {
- struct callback_head *q = last, *p = q->next;
- while (1) {
- if (p->func == func) {
- q->next = p->next;
- if (p == last)
- task->task_works = q == p ? NULL : q;
- res = p;
- break;
- }
- if (p == last)
- break;
- q = p;
- p = q->next;
+ struct callback_head **workp, *work;
+
+again:
+ workp = &task->task_works;
+ work = *workp;
+ while (work) {
+ if (work->func == func) {
+ if (cmpxchg(workp, work, work->next) == work)
+ return work;
+ goto again;
}
+
+ workp = &work->next;
+ work = *workp;
}
- raw_spin_unlock_irqrestore(&task->pi_lock, flags);
- return res;
+
+ return NULL;
}
-void task_work_run(void)
+static void __task_work_run(struct callback_head *tail)
{
- struct task_struct *task = current;
- struct callback_head *p, *q;
-
- while (1) {
- raw_spin_lock_irq(&task->pi_lock);
- p = task->task_works;
- task->task_works = NULL;
- raw_spin_unlock_irq(&task->pi_lock);
-
- if (unlikely(!p))
- return;
-
- q = p->next; /* head */
- p->next = NULL; /* cut it */
- while (q) {
- p = q->next;
- q->func(q);
- q = p;
+ struct callback_head **head = ¤t->task_works;
+
+ do {
+ struct callback_head *work = xchg(head, NULL);
+ while (work) {
+ struct callback_head *next = ACCESS_ONCE(work->next);
+
+ WARN_ON_ONCE(work == &dead);
+
+ work->func(work);
+ work = next;
}
- }
+ } while (cmpxchg(head, NULL, tail) != NULL);
+}
+
+void task_work_run(void)
+{
+ __task_work_run(NULL);
+}
+
+void task_work_exit(void)
+{
+ __task_work_run(&dead);
}
--
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