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From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Jens Axboe" <axboe@kernel.dk>,
	"Carter Li 李通洲" <carter.li@eoitek.com>,
	"Pavel Begunkov" <asml.silence@gmail.com>,
	io-uring <io-uring@vger.kernel.org>
Subject: Re: [ISSUE] The time cost of IOSQE_IO_LINK
Date: Tue, 18 Feb 2020 15:56:46 +0100
Message-ID: <20200218145645.GB3466@redhat.com> (raw)
In-Reply-To: <20200218131310.GZ14914@hirez.programming.kicks-ass.net>

On 02/18, Peter Zijlstra wrote:
>
> But this has me wondering about task_work_run(), as it is it will
> unconditionally take pi_lock,

because spin_unlock_wait() was removed ;) task_work_run() doesn't
really need to take pi_lock at all. 

> would not something like this make sense?

I think yes, but see below.

> --- a/kernel/task_work.c
> +++ b/kernel/task_work.c
> @@ -93,16 +93,20 @@ void task_work_run(void)
>  	struct callback_head *work, *head, *next;
>
>  	for (;;) {
> +		work = READ_ONCE(task->task_work);
> +		if (!work)
> +			break

This is wrong if PF_EXITING is set, in this case we must set
task->task_works = work_exited.

> +
>  		/*
>  		 * work->func() can do task_work_add(), do not set
>  		 * work_exited unless the list is empty.
>  		 */
>  		raw_spin_lock_irq(&task->pi_lock);
>  		do {
> -			work = READ_ONCE(task->task_works);
> -			head = !work && (task->flags & PF_EXITING) ?
> -				&work_exited : NULL;
> -		} while (cmpxchg(&task->task_works, work, head) != work);
> +			head = NULL;
> +			if (unlikely(!work && (task->flags & PF_EXITING)))
> +				head = &work_exited;
> +		} while (!try_cmpxchg(&task->task_works, &work, head));
>  		raw_spin_unlock_irq(&task->pi_lock);
>
>  		if (!work)

otherwise I think this is correct, but how about the patch below?
Then this code can be changed to use try_cmpxchg().

Oleg.

--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -97,17 +97,24 @@ void task_work_run(void)
 		 * work->func() can do task_work_add(), do not set
 		 * work_exited unless the list is empty.
 		 */
-		raw_spin_lock_irq(&task->pi_lock);
 		do {
+			head = NULL;
 			work = READ_ONCE(task->task_works);
-			head = !work && (task->flags & PF_EXITING) ?
-				&work_exited : NULL;
+			if (!work) {
+				if (task->flags & PF_EXITING)
+					head = &work_exited;
+				else
+					break;
+			}
 		} while (cmpxchg(&task->task_works, work, head) != work);
-		raw_spin_unlock_irq(&task->pi_lock);
 
 		if (!work)
 			break;
 
+		// Synchronize with task_work_cancel()
+		raw_spin_lock_irq(&task->pi_lock);
+		raw_spin_unlock_irq(&task->pi_lock);
+
 		do {
 			next = work->next;
 			work->func(work);


  parent reply index

Thread overview: 59+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-12 16:31 Carter Li 李通洲
2020-02-12 17:11 ` Jens Axboe
2020-02-12 17:22   ` Jens Axboe
2020-02-12 17:29     ` Jens Axboe
2020-02-13  0:33   ` Carter Li 李通洲
2020-02-13 15:08     ` Pavel Begunkov
2020-02-13 15:14       ` Jens Axboe
2020-02-13 15:51         ` Carter Li 李通洲
2020-02-14  1:25           ` Carter Li 李通洲
2020-02-14  2:45             ` Jens Axboe
2020-02-14  5:03               ` Jens Axboe
2020-02-14 15:32                 ` Peter Zijlstra
2020-02-14 15:47                   ` Jens Axboe
2020-02-14 16:18                     ` Jens Axboe
2020-02-14 17:52                       ` Jens Axboe
2020-02-14 20:44                         ` Jens Axboe
2020-02-15  0:16                           ` Carter Li 李通洲
2020-02-15  1:10                             ` Jens Axboe
2020-02-15  1:25                               ` Carter Li 李通洲
2020-02-15  1:27                                 ` Jens Axboe
2020-02-15  6:01                                   ` Jens Axboe
2020-02-15  6:32                                     ` Carter Li 李通洲
2020-02-15 15:11                                       ` Jens Axboe
2020-02-16 19:06                                     ` Pavel Begunkov
2020-02-16 22:23                                       ` Jens Axboe
2020-02-17 10:30                                         ` Pavel Begunkov
2020-02-17 19:30                                           ` Jens Axboe
2020-02-16 23:06                                       ` Jens Axboe
2020-02-16 23:07                                         ` Jens Axboe
2020-02-17 12:09                           ` Peter Zijlstra
2020-02-17 16:12                             ` Jens Axboe
2020-02-17 17:16                               ` Jens Axboe
2020-02-17 17:46                                 ` Peter Zijlstra
2020-02-17 18:16                                   ` Jens Axboe
2020-02-18 13:13                                     ` Peter Zijlstra
2020-02-18 14:27                                       ` [PATCH] asm-generic/atomic: Add try_cmpxchg() fallbacks Peter Zijlstra
2020-02-18 14:40                                         ` Peter Zijlstra
2020-02-20 10:30                                         ` Will Deacon
2020-02-20 10:37                                           ` Peter Zijlstra
2020-02-20 10:39                                             ` Will Deacon
2020-02-18 14:56                                       ` Oleg Nesterov [this message]
2020-02-18 15:07                                         ` [ISSUE] The time cost of IOSQE_IO_LINK Oleg Nesterov
2020-02-18 15:38                                           ` Peter Zijlstra
2020-02-18 16:33                                             ` Jens Axboe
2020-02-18 15:07                                         ` Peter Zijlstra
2020-02-18 15:50                                           ` [PATCH] task_work_run: don't take ->pi_lock unconditionally Oleg Nesterov
2020-02-20 16:39                                             ` Peter Zijlstra
2020-02-20 17:22                                               ` Oleg Nesterov
2020-02-20 17:49                                                 ` Peter Zijlstra
2020-02-21 14:52                                                   ` Oleg Nesterov
2020-02-24 18:47                                                     ` Jens Axboe
2020-02-28 19:17                                                       ` Jens Axboe
2020-02-28 19:25                                                         ` Peter Zijlstra
2020-02-28 19:28                                                           ` Jens Axboe
2020-02-28 20:06                                                             ` Peter Zijlstra
2020-02-28 20:15                                                               ` Jens Axboe
2020-02-18 16:46                                       ` [ISSUE] The time cost of IOSQE_IO_LINK Jens Axboe
2020-02-18 16:52                                         ` Jens Axboe
2020-02-18 13:13                               ` Peter Zijlstra

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