task_work: only grab task signal lock when needed

If JOBCTL_TASK_WORK is already set on the targeted task, then we need
not go through {lock,unlock}_task_sighand() to set it again and queue
a signal wakeup. This is safe as we're checking it _after_ adding the
new task_work with cmpxchg().

The ordering is as follows:

task_work_add()				get_signal()
--------------------------------------------------------------
STORE(task->task_works, new_work);	STORE(task->jobctl);
mb();					mb();
LOAD(task->jobctl);			LOAD(task->task_works);

This speeds up TWA_SIGNAL handling quite a bit, which is important now
that io_uring is relying on it for all task_work deliveries.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Jann Horn <jannh@google.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/kernel/signal.c b/kernel/signal.c
index 6f16f7c..42b67d2 100644 (file)
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2541,7 +2541,21 @@ bool get_signal(struct ksignal *ksig)
 
 relock:
        spin_lock_irq(&sighand->siglock);
-       current->jobctl &= ~JOBCTL_TASK_WORK;
+       /*
+        * Make sure we can safely read ->jobctl() in task_work add. As Oleg
+        * states:
+        *
+        * It pairs with mb (implied by cmpxchg) before READ_ONCE. So we
+        * roughly have
+        *
+        *      task_work_add:                          get_signal:
+        *      STORE(task->task_works, new_work);      STORE(task->jobctl);
+        *      mb();                                   mb();
+        *      LOAD(task->jobctl);                     LOAD(task->task_works);
+        *
+        * and we can rely on STORE-MB-LOAD [ in task_work_add].
+        */
+       smp_store_mb(current->jobctl, current->jobctl & ~JOBCTL_TASK_WORK);
        if (unlikely(current->task_works)) {
                spin_unlock_irq(&sighand->siglock);
                task_work_run();

diff --git a/kernel/task_work.c b/kernel/task_work.c
index 5c0848c..613b2d6 100644 (file)
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -42,7 +42,13 @@ task_work_add(struct task_struct *task, struct callback_head *work, int notify)
                set_notify_resume(task);
                break;
        case TWA_SIGNAL:
-               if (lock_task_sighand(task, &flags)) {
+               /*
+                * Only grab the sighand lock if we don't already have some
+                * task_work pending. This pairs with the smp_store_mb()
+                * in get_signal(), see comment there.
+                */
+               if (!(READ_ONCE(task->jobctl) & JOBCTL_TASK_WORK) &&
+                   lock_task_sighand(task, &flags)) {
                        task->jobctl |= JOBCTL_TASK_WORK;
                        signal_wake_up(task, 0);
                        unlock_task_sighand(task, &flags);