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From: Oleg Nesterov <[email protected]>
To: Jann Horn <[email protected]>
Cc: Jens Axboe <[email protected]>,
	Peter Zijlstra <[email protected]>,
	io-uring <[email protected]>,
	stable <[email protected]>, Josef <[email protected]>
Subject: Re: [PATCH 2/2] io_uring: use TWA_SIGNAL for task_work if the task isn't running
Date: Tue, 11 Aug 2020 08:45:16 +0200	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAG48ez2EzOpWZbhnuBxVBXjRbLZULJJeeTBsdbL6Hzh9-1YYhA@mail.gmail.com>

On 08/11, Jann Horn wrote:
>
> > --- a/kernel/task_work.c
> > +++ b/kernel/task_work.c
> > @@ -42,7 +42,8 @@ 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)) {
> > +               if (!(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);
> 
> I think that should work in theory, but if you want to be able to do a
> proper unlocked read of task->jobctl here, then I think you'd have to
> use READ_ONCE() here

Agreed,

> and make all existing writes to ->jobctl use
> WRITE_ONCE().

->jobctl is always modified with ->siglock held, do we really need
WRITE_ONCE() ?

> Also, I think that to make this work, stuff like get_signal() will
> need to use memory barriers to ensure that reads from ->task_works are
> ordered after ->jobctl has been cleared

Why? I don't follow.

Afaics, we only need to ensure that task_work_add() checks JOBCTL_TASK_WORK
after it adds the new work to the ->task_works list, and we can rely on
cmpxchg() ?

Oleg.


  parent reply	other threads:[~2020-08-11  6:45 UTC|newest]

Thread overview: 41+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-08 18:34 [PATCHSET 0/2] io_uring: use TWA_SIGNAL more carefully Jens Axboe
2020-08-08 18:34 ` [PATCH 1/2] kernel: split task_work_add() into two separate helpers Jens Axboe
2020-08-10 11:37   ` peterz
2020-08-10 15:01     ` Jens Axboe
2020-08-10 15:28       ` peterz
2020-08-10 17:51       ` Jens Axboe
2020-08-10 19:53         ` Peter Zijlstra
2020-08-08 18:34 ` [PATCH 2/2] io_uring: use TWA_SIGNAL for task_work if the task isn't running Jens Axboe
2020-08-10 11:42   ` peterz
2020-08-10 15:02     ` Jens Axboe
2020-08-10 19:21       ` Jens Axboe
2020-08-10 20:12         ` Peter Zijlstra
2020-08-10 20:13           ` Jens Axboe
2020-08-10 20:25             ` Jens Axboe
2020-08-10 20:32               ` Peter Zijlstra
2020-08-10 20:35                 ` Jens Axboe
2020-08-10 20:35               ` Jann Horn
2020-08-10 21:06                 ` Jens Axboe
2020-08-10 21:10                   ` Peter Zijlstra
2020-08-10 21:12                     ` Jens Axboe
2020-08-10 21:26                       ` Jann Horn
2020-08-10 21:28                         ` Jens Axboe
2020-08-10 22:01                           ` Jens Axboe
2020-08-10 22:41                             ` Jann Horn
2020-08-11  1:25                               ` Jens Axboe
2020-08-11  6:45                               ` Oleg Nesterov [this message]
2020-08-11  6:56                                 ` Peter Zijlstra
2020-08-11  7:14                                   ` Oleg Nesterov
2020-08-11  7:26                                     ` Oleg Nesterov
2020-08-11  7:49                                       ` Peter Zijlstra
2020-08-11  7:45                                     ` Peter Zijlstra
2020-08-11  8:10                                       ` Oleg Nesterov
2020-08-11 13:06                                         ` Jens Axboe
2020-08-11 14:05                                           ` Oleg Nesterov
2020-08-11 14:12                                             ` Jens Axboe
2020-08-10 21:27                       ` Jens Axboe
2020-08-10 20:16           ` Jens Axboe
2020-08-13 16:25   ` Sasha Levin
2020-08-19 23:57   ` Sasha Levin
2020-08-19 23:59     ` Jens Axboe
2020-08-20  0:02       ` Jens Axboe

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