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authorJens Axboe <axboe@kernel.dk>2020-08-13 09:01:38 -0600
committerJens Axboe <axboe@kernel.dk>2020-08-13 09:01:38 -0600
commitebf0d100df0731901c16632f78d78d35f4123bc4 (patch)
tree75defc3ec0621de7306f54a658c8797b34cab263 /kernel/task_work.c
parentf254ac04c8744cf7bfed012717eac34eacc65dfb (diff)
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>
Diffstat (limited to 'kernel/task_work.c')
-rw-r--r--kernel/task_work.c8
1 files changed, 7 insertions, 1 deletions
diff --git a/kernel/task_work.c b/kernel/task_work.c
index 5c0848ca1287..613b2d634af8 100644
--- 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);