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Subject[PATCH v2 1/2] task_work_queue: add generic process-context callbacks
Provide a simple mechanism that allows running code in the
(nonatomic) context of the arbitrary task.

The caller does task_work_queue(task, task_work) and this task
executes task_work->func() either from do_notify_resume() or
from do_exit(). The callback can rely on PF_EXITING to detect
the latter case.

"struct task_work" can be embedded in another struct, still it
has "void *data" to handle the most common/simple case.

This allows us to kill the ->replacement_session_keyring hack,
and potentially this can have more users.

Performance-wise, this adds 2 "unlikely(!hlist_empty())" checks
into tracehook_notify_resume() and do_exit(). But at the same
time we can remove the "replacement_session_keyring != NULL"
checks from arch/*/signal.c and exit_creds().
Note: task_work_queue/task_work_run abuses ->pi_lock. This is
only because this lock is already used by lookup_pi_state() to
synchronize with do_exit() setting PF_EXITING. Fortunately the
scope of this lock in task_work.c is really tiny, and the code
is unlikely anyway.

Todo:
- move clear_thread_flag(TIF_NOTIFY_RESUME) from arch/
to tracehook_notify_resume()

- rename tracehook_notify_resume() and move it into
linux/task_work.h

- m68k and xtensa don't have TIF_NOTIFY_RESUME and thus
they can't use this feature.

However, ->replacement_session_keyring equally needs
this logic, task_work_queue() is not worse.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
include/linux/sched.h | 2 +
include/linux/task_work.h | 48 ++++++++++++++++++++++++++++
include/linux/tracehook.h | 10 +++++-
kernel/Makefile | 2 +-
kernel/exit.c | 5 ++-
kernel/fork.c | 1 +
kernel/task_work.c | 77 +++++++++++++++++++++++++++++++++++++++++++++
7 files changed, 142 insertions(+), 3 deletions(-)
create mode 100644 include/linux/task_work.h
create mode 100644 kernel/task_work.c
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 81a173c..be004ac 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1445,6 +1445,8 @@ struct task_struct {
int (*notifier)(void *priv);
void *notifier_data;
sigset_t *notifier_mask;
+ struct hlist_head task_works;
+
struct audit_context *audit_context;
#ifdef CONFIG_AUDITSYSCALL
uid_t loginuid;
diff --git a/include/linux/task_work.h b/include/linux/task_work.h
new file mode 100644
index 0000000..141a2b5
--- /dev/null
+++ b/include/linux/task_work.h
@@ -0,0 +1,48 @@
+#ifndef _LINUX_TASK_WORK_H
+#define _LINUX_TASK_WORK_H 1
+
+#include <linux/sched.h>
+
+struct task_work;
+typedef void (*task_work_func_t)(struct task_work *);
+
+struct task_work {
+ struct hlist_node hlist;
+ task_work_func_t func;
+ void *data;
+};
+
+static inline void
+init_task_work(struct task_work *twork, task_work_func_t func, void *data)
+{
+ twork->func = func;
+ twork->data = data;
+}
+
+#ifdef TIF_NOTIFY_RESUME
+int task_work_queue(struct task_struct *task, struct task_work *twork);
+struct task_work *task_work_cancel(struct task_struct *, task_work_func_t);
+void task_work_run(struct task_struct *task);
+#else
+static inline int
+task_work_queue(struct task_struct *task, struct task_work *twork)
+{
+ return -ENOTSUPP;
+}
+static inline struct task_work *
+task_work_cancel(struct task_struct *task, task_work_func_t func)
+{
+ return NULL;
+}
+static inline void task_work_run(struct task_struct *task)
+{
+}
+#endif /* TIF_NOTIFY_RESUME */
+
+static inline void exit_task_work(struct task_struct *task)
+{
+ if (unlikely(!hlist_empty(&task->task_works)))
+ task_work_run(task);
+}
+
+#endif /* _LINUX_TASK_WORK_H */
diff --git a/include/linux/tracehook.h b/include/linux/tracehook.h
index 51bd91d..67dd262 100644
--- a/include/linux/tracehook.h
+++ b/include/linux/tracehook.h
@@ -46,7 +46,7 @@
#ifndef _LINUX_TRACEHOOK_H
#define _LINUX_TRACEHOOK_H 1

-#include <linux/sched.h>
+#include <linux/task_work.h>
#include <linux/ptrace.h>
#include <linux/security.h>
struct linux_binprm;
@@ -184,6 +184,14 @@ static inline void set_notify_resume(struct task_struct *task)
*/
static inline void tracehook_notify_resume(struct pt_regs *regs)
{
+ /*
+ * The caller just cleared TIF_NOTIFY_RESUME. This barrier
+ * pairs with task_work_queue()->set_notify_resume() after
+ * hlist_add_head(task->task_works);
+ */
+ smp_mb__after_clear_bit();
+ if (unlikely(!hlist_empty(&current->task_works)))
+ task_work_run(current);
}
#endif /* TIF_NOTIFY_RESUME */

diff --git a/kernel/Makefile b/kernel/Makefile
index cb41b95..5790f8b 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -10,7 +10,7 @@ obj-y = fork.o exec_domain.o panic.o printk.o \
kthread.o wait.o kfifo.o sys_ni.o posix-cpu-timers.o mutex.o \
hrtimer.o rwsem.o nsproxy.o srcu.o semaphore.o \
notifier.o ksysfs.o cred.o \
- async.o range.o groups.o
+ async.o range.o groups.o task_work.o

ifdef CONFIG_FUNCTION_TRACER
# Do not trace debug files and internal ftrace files
diff --git a/kernel/exit.c b/kernel/exit.c
index d8bd3b4..dc852c2 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -946,11 +946,14 @@ void do_exit(long code)
exit_signals(tsk); /* sets PF_EXITING */
/*
* tsk->flags are checked in the futex code to protect against
- * an exiting task cleaning up the robust pi futexes.
+ * an exiting task cleaning up the robust pi futexes, and in
+ * task_work_queue() to avoid the race with exit_task_work().
*/
smp_mb();
raw_spin_unlock_wait(&tsk->pi_lock);

+ exit_task_work(tsk);
+
exit_irq_thread();

if (unlikely(in_atomic()))
diff --git a/kernel/fork.c b/kernel/fork.c
index b9372a0..d1108ac 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1380,6 +1380,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
*/
p->group_leader = p;
INIT_LIST_HEAD(&p->thread_group);
+ INIT_HLIST_HEAD(&p->task_works);

/* Now that the task is set up, run cgroup callbacks if
* necessary. We need to run them before the task is visible
diff --git a/kernel/task_work.c b/kernel/task_work.c
new file mode 100644
index 0000000..a721a5e
--- /dev/null
+++ b/kernel/task_work.c
@@ -0,0 +1,77 @@
+#include <linux/tracehook.h>
+
+#ifdef TIF_NOTIFY_RESUME
+int task_work_queue(struct task_struct *task, struct task_work *twork)
+{
+ unsigned long flags;
+ int err = -ESRCH;
+ /*
+ * We must not insert the new work if the task has already passed
+ * exit_task_work(). We rely on do_exit()->raw_spin_unlock_wait()
+ * and check PF_EXITING under pi_lock.
+ */
+ raw_spin_lock_irqsave(&task->pi_lock, flags);
+ if (likely(!(task->flags & PF_EXITING))) {
+ hlist_add_head(&twork->hlist, &task->task_works);
+ err = 0;
+ }
+ raw_spin_unlock_irqrestore(&task->pi_lock, flags);
+
+ /* test_and_set_bit() implies mb(), see tracehook_notify_resume(). */
+ if (likely(!err))
+ set_notify_resume(task);
+ return err;
+}
+
+struct task_work *
+task_work_cancel(struct task_struct *task, task_work_func_t func)
+{
+ unsigned long flags;
+ struct task_work *twork;
+ struct hlist_node *pos;
+
+ raw_spin_lock_irqsave(&task->pi_lock, flags);
+ hlist_for_each_entry(twork, pos, &task->task_works, hlist) {
+ if (twork->func == func) {
+ hlist_del(&twork->hlist);
+ goto found;
+ }
+ }
+ twork = NULL;
+ found:
+ raw_spin_unlock_irqrestore(&task->pi_lock, flags);
+
+ return twork;
+}
+
+void task_work_run(struct task_struct *task)
+{
+ struct hlist_head task_works;
+ struct hlist_node *pos;
+
+ raw_spin_lock_irq(&task->pi_lock);
+ hlist_move_list(&task->task_works, &task_works);
+ raw_spin_unlock_irq(&task->pi_lock);
+
+ if (unlikely(hlist_empty(&task_works)))
+ return;
+ /*
+ * We use hlist to save the space in task_struct, but we want fifo.
+ * Find the last entry, the list should be short, then process them
+ * in reverse order.
+ */
+ for (pos = task_works.first; pos->next; pos = pos->next)
+ ;
+
+ for (;;) {
+ struct hlist_node **pprev = pos->pprev;
+ struct task_work *twork = container_of(pos, struct task_work,
+ hlist);
+ twork->func(twork);
+
+ if (pprev == &task_works.first)
+ break;
+ pos = container_of(pprev, struct hlist_node, next);
+ }
+}
+#endif /* TIF_NOTIFY_RESUME */
--
1.5.5.1



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