From: Oleg Nesterov <oleg@redhat.com>
To: Jens Axboe <axboe@kernel.dk>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Jann Horn <jannh@google.com>
Subject: Re: [PATCH] task_work: only grab task signal lock when needed
Date: Tue, 11 Aug 2020 17:23:10 +0200 [thread overview]
Message-ID: <20200811152310.GF21797@redhat.com> (raw)
In-Reply-To: <0028d3ea-4d05-405f-b457-75c83d381d89@kernel.dk>
On 08/11, Jens Axboe 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 (!(READ_ONCE(task->jobctl) & JOBCTL_TASK_WORK) &&
> + lock_task_sighand(task, &flags)) {
Aaaaah, sorry Jens, now I think this is racy. So I am glad I didn't add
this optimization into the initial version ;)
It is possible that JOBCTL_TASK_WORK is set but ->task_works == NULL. Say,
task_work_add(TWA_SIGNAL) + task_work_cancel(), or the target task can call
task_work_run() before it enters get_signal().
And in this case another task_work_add(tsk, TWA_SIGNAL) can actually race
with get_signal() which does
current->jobctl &= ~JOBCTL_TASK_WORK;
if (unlikely(current->task_works)) {
spin_unlock_irq(&sighand->siglock);
task_work_run();
nothing guarantees that get_signal() sees ->task_works != NULL. Probably
this is what Jann meant.
We can probably add a barrier into get_signal() but I didn't sleep today,
I'll try to think tomorrow.
Oleg.
next prev parent reply other threads:[~2020-08-11 15:23 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-11 14:25 [PATCH] task_work: only grab task signal lock when needed Jens Axboe
2020-08-11 15:23 ` Oleg Nesterov [this message]
2020-08-11 16:45 ` Jens Axboe
2020-08-12 14:54 ` Oleg Nesterov
2020-08-12 20:06 ` Peter Zijlstra
2020-08-12 23:13 ` Jens Axboe
2020-08-13 11:48 ` Oleg Nesterov
-- strict thread matches above, loose matches on Subject: below --
2020-08-13 15:07 Jens Axboe
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20200811152310.GF21797@redhat.com \
--to=oleg@redhat.com \
--cc=axboe@kernel.dk \
--cc=jannh@google.com \
--cc=linux-kernel@vger.kernel.org \
--cc=peterz@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.