[GIT PULL] core kernel fixes for v3.7

From: Ingo Molnar
Date: Mon Oct 01 2012 - 08:27:43 EST

Linus,

Please pull the latest core-urgent-for-linus git tree from:

git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git core-urgent-for-linus

HEAD: f784e8a7989c0da3062d04bfea3db90f41e8f738 task_work: Simplify the usage in ptrace_notify() and get_signal_to_deliver()

This is a complex task_work series from Oleg that fixes the bug
that this VFS commit tried to fix:

d35abdb28824 hold task_lock around checks in keyctl

but solves the problem without the lockup regression that
d35abdb28824 introduced in v3.6.

This series came late in v3.6 and I did not feel confident about
it so late in the cycle. Might be worth backporting to -stable
if it proves itself upstream.

Thanks,

Ingo

------------------>
Oleg Nesterov (4):
task_work: Make task_work_add() lockless
task_work: task_work_add() should not succeed after exit_task_work()
task_work: Revert "hold task_lock around checks in keyctl"
task_work: Simplify the usage in ptrace_notify() and get_signal_to_deliver()


include/linux/task_work.h | 3 +-
kernel/signal.c | 18 ++------
kernel/task_work.c | 111 +++++++++++++++++++++++++---------------------
security/keys/keyctl.c | 2 -
4 files changed, 66 insertions(+), 68 deletions(-)

diff --git a/include/linux/task_work.h b/include/linux/task_work.h
index fb46b03..ca5a1cf 100644
--- a/include/linux/task_work.h
+++ b/include/linux/task_work.h
@@ -18,8 +18,7 @@ void task_work_run(void);

static inline void exit_task_work(struct task_struct *task)
{
- if (unlikely(task->task_works))
- task_work_run();
+ task_work_run();
}

#endif /* _LINUX_TASK_WORK_H */
diff --git a/kernel/signal.c b/kernel/signal.c
index be4f856..2c681f1 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1971,13 +1971,8 @@ static void ptrace_do_notify(int signr, int exit_code, int why)
void ptrace_notify(int exit_code)
{
BUG_ON((exit_code & (0x7f | ~0xffff)) != SIGTRAP);
- if (unlikely(current->task_works)) {
- if (test_and_clear_ti_thread_flag(current_thread_info(),
- TIF_NOTIFY_RESUME)) {
- smp_mb__after_clear_bit();
- task_work_run();
- }
- }
+ if (unlikely(current->task_works))
+ task_work_run();

spin_lock_irq(&current->sighand->siglock);
ptrace_do_notify(SIGTRAP, exit_code, CLD_TRAPPED);
@@ -2198,13 +2193,8 @@ int get_signal_to_deliver(siginfo_t *info, struct k_sigaction *return_ka,
struct signal_struct *signal = current->signal;
int signr;

- if (unlikely(current->task_works)) {
- if (test_and_clear_ti_thread_flag(current_thread_info(),
- TIF_NOTIFY_RESUME)) {
- smp_mb__after_clear_bit();
- task_work_run();
- }
- }
+ if (unlikely(current->task_works))
+ task_work_run();

if (unlikely(uprobe_deny_signal()))
return 0;
diff --git a/kernel/task_work.c b/kernel/task_work.c
index d320d44..65bd3c9 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -2,26 +2,20 @@
#include <linux/task_work.h>
#include <linux/tracehook.h>

+static struct callback_head work_exited; /* all we need is ->next == NULL */
+
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;
+ struct callback_head *head;

- /*
- * 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);
+ do {
+ head = ACCESS_ONCE(task->task_works);
+ if (unlikely(head == &work_exited))
+ return -ESRCH;
+ work->next = head;
+ } while (cmpxchg(&task->task_works, head, work) != head);

- /* test_and_set_bit() implies mb(), see tracehook_notify_resume(). */
if (notify)
set_notify_resume(task);
return 0;
@@ -30,52 +24,69 @@ task_work_add(struct task_struct *task, struct callback_head *twork, bool notify
struct callback_head *
task_work_cancel(struct task_struct *task, task_work_func_t func)
{
+ struct callback_head **pprev = &task->task_works;
+ struct callback_head *work = NULL;
unsigned long flags;
- struct callback_head *last, *res = NULL;
-
+ /*
+ * If cmpxchg() fails we continue without updating pprev.
+ * Either we raced with task_work_add() which added the
+ * new entry before this work, we will find it again. Or
+ * we raced with task_work_run(), *pprev == NULL/exited.
+ */
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;
- }
+ while ((work = ACCESS_ONCE(*pprev))) {
+ read_barrier_depends();
+ if (work->func != func)
+ pprev = &work->next;
+ else if (cmpxchg(pprev, work, work->next) == work)
+ break;
}
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
- return res;
+
+ return work;
}

void task_work_run(void)
{
struct task_struct *task = current;
- struct callback_head *p, *q;
+ struct callback_head *work, *head, *next;
+
+ for (;;) {
+ /*
+ * work->func() can do task_work_add(), do not set
+ * work_exited unless the list is empty.
+ */
+ do {
+ work = ACCESS_ONCE(task->task_works);
+ head = !work && (task->flags & PF_EXITING) ?
+ &work_exited : NULL;
+ } while (cmpxchg(&task->task_works, work, head) != work);

- 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 (!work)
+ break;
+ /*
+ * Synchronize with task_work_cancel(). It can't remove
+ * the first entry == work, cmpxchg(task_works) should
+ * fail, but it can play with *work and other entries.
+ */
+ raw_spin_unlock_wait(&task->pi_lock);
+ smp_mb();

- if (unlikely(!p))
- return;
+ /* Reverse the list to run the works in fifo order */
+ head = NULL;
+ do {
+ next = work->next;
+ work->next = head;
+ head = work;
+ work = next;
+ } while (work);

- q = p->next; /* head */
- p->next = NULL; /* cut it */
- while (q) {
- p = q->next;
- q->func(q);
- q = p;
+ work = head;
+ do {
+ next = work->next;
+ work->func(work);
+ work = next;
cond_resched();
- }
+ } while (work);
}
}
diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index 3364fbf..6cfc647 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -1486,7 +1486,6 @@ long keyctl_session_to_parent(void)
oldwork = NULL;
parent = me->real_parent;

- task_lock(parent);
/* the parent mustn't be init and mustn't be a kernel thread */
if (parent->pid <= 1 || !parent->mm)
goto unlock;
@@ -1530,7 +1529,6 @@ long keyctl_session_to_parent(void)
if (!ret)
newwork = NULL;
unlock:
- task_unlock(parent);
write_unlock_irq(&tasklist_lock);
rcu_read_unlock();
if (oldwork)
--
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