[PATCH 5.7 390/393] task_work: only grab task signal lock when needed
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STALE2241d
REVIEWED: 2 (2M)
From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Date: 2020-08-17 18:36:06
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1 review trailer.
From: Jens Axboe <axboe@kernel.dk>
commit ebf0d100df0731901c16632f78d78d35f4123bc4 upstream.
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/signal.c | 16 +++++++++++++++-
kernel/task_work.c | 8 +++++++-
2 files changed, 22 insertions(+), 2 deletions(-)
--- 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(); --- a/kernel/task_work.c +++ b/kernel/task_work.c
@@ -42,7 +42,13 @@ task_work_add(struct task_struct *task, 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);