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Subject[PATCH 1/2] task_work: teach task_work_add() to do signal_wake_up()
Date
From: Oleg Nesterov <oleg@redhat.com>

So that the target task will exit the wait_event_interruptible-like
loop and call task_work_run() asap.

The patch turns "bool notify" into 0,TWA_RESUME,TWA_SIGNAL enum, the
new TWA_SIGNAL flag implies signal_wake_up(). However, it needs to
avoid the race with recalc_sigpending(), so the patch also adds the
new JOBCTL_TASK_WORK bit included in JOBCTL_PENDING_MASK.

TODO: once this patch is merged we need to change all current users
of task_work_add(notify = true) to use TWA_RESUME.

Cc: stable@vger.kernel.org # v5.7
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
include/linux/sched/jobctl.h | 4 +++-
include/linux/task_work.h | 5 ++++-
kernel/signal.c | 10 +++++++---
kernel/task_work.c | 16 ++++++++++++++--
4 files changed, 28 insertions(+), 7 deletions(-)

diff --git a/include/linux/sched/jobctl.h b/include/linux/sched/jobctl.h
index fa067de9f1a9..d2b4204ba4d3 100644
--- a/include/linux/sched/jobctl.h
+++ b/include/linux/sched/jobctl.h
@@ -19,6 +19,7 @@ struct task_struct;
#define JOBCTL_TRAPPING_BIT 21 /* switching to TRACED */
#define JOBCTL_LISTENING_BIT 22 /* ptracer is listening for events */
#define JOBCTL_TRAP_FREEZE_BIT 23 /* trap for cgroup freezer */
+#define JOBCTL_TASK_WORK_BIT 24 /* set by TWA_SIGNAL */

#define JOBCTL_STOP_DEQUEUED (1UL << JOBCTL_STOP_DEQUEUED_BIT)
#define JOBCTL_STOP_PENDING (1UL << JOBCTL_STOP_PENDING_BIT)
@@ -28,9 +29,10 @@ struct task_struct;
#define JOBCTL_TRAPPING (1UL << JOBCTL_TRAPPING_BIT)
#define JOBCTL_LISTENING (1UL << JOBCTL_LISTENING_BIT)
#define JOBCTL_TRAP_FREEZE (1UL << JOBCTL_TRAP_FREEZE_BIT)
+#define JOBCTL_TASK_WORK (1UL << JOBCTL_TASK_WORK_BIT)

#define JOBCTL_TRAP_MASK (JOBCTL_TRAP_STOP | JOBCTL_TRAP_NOTIFY)
-#define JOBCTL_PENDING_MASK (JOBCTL_STOP_PENDING | JOBCTL_TRAP_MASK)
+#define JOBCTL_PENDING_MASK (JOBCTL_STOP_PENDING | JOBCTL_TRAP_MASK | JOBCTL_TASK_WORK)

extern bool task_set_jobctl_pending(struct task_struct *task, unsigned long mask);
extern void task_clear_jobctl_trapping(struct task_struct *task);
diff --git a/include/linux/task_work.h b/include/linux/task_work.h
index bd9a6a91c097..0fb93aafa478 100644
--- a/include/linux/task_work.h
+++ b/include/linux/task_work.h
@@ -13,7 +13,10 @@ init_task_work(struct callback_head *twork, task_work_func_t func)
twork->func = func;
}

-int task_work_add(struct task_struct *task, struct callback_head *twork, bool);
+#define TWA_RESUME 1
+#define TWA_SIGNAL 2
+int task_work_add(struct task_struct *task, struct callback_head *twork, int);
+
struct callback_head *task_work_cancel(struct task_struct *, task_work_func_t);
void task_work_run(void);

diff --git a/kernel/signal.c b/kernel/signal.c
index 5ca48cc5da76..ee22ec78fd6d 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2529,9 +2529,6 @@ bool get_signal(struct ksignal *ksig)
struct signal_struct *signal = current->signal;
int signr;

- if (unlikely(current->task_works))
- task_work_run();
-
if (unlikely(uprobe_deny_signal()))
return false;

@@ -2544,6 +2541,13 @@ bool get_signal(struct ksignal *ksig)

relock:
spin_lock_irq(&sighand->siglock);
+ current->jobctl &= ~JOBCTL_TASK_WORK;
+ if (unlikely(current->task_works)) {
+ spin_unlock_irq(&sighand->siglock);
+ task_work_run();
+ goto relock;
+ }
+
/*
* Every stopped thread goes here after wakeup. Check to see if
* we should notify the parent, prepare_signal(SIGCONT) encodes
diff --git a/kernel/task_work.c b/kernel/task_work.c
index 825f28259a19..5c0848ca1287 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -25,9 +25,10 @@ static struct callback_head work_exited; /* all we need is ->next == NULL */
* 0 if succeeds or -ESRCH.
*/
int
-task_work_add(struct task_struct *task, struct callback_head *work, bool notify)
+task_work_add(struct task_struct *task, struct callback_head *work, int notify)
{
struct callback_head *head;
+ unsigned long flags;

do {
head = READ_ONCE(task->task_works);
@@ -36,8 +37,19 @@ task_work_add(struct task_struct *task, struct callback_head *work, bool notify)
work->next = head;
} while (cmpxchg(&task->task_works, head, work) != head);

- if (notify)
+ switch (notify) {
+ case TWA_RESUME:
set_notify_resume(task);
+ break;
+ case TWA_SIGNAL:
+ if (lock_task_sighand(task, &flags)) {
+ task->jobctl |= JOBCTL_TASK_WORK;
+ signal_wake_up(task, 0);
+ unlock_task_sighand(task, &flags);
+ }
+ break;
+ }
+
return 0;
}

--
2.27.0
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