linux-kernel - Re: [PATCH v11 8/8] vhost: use vhost

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Message-ID: <20230516183757.GA1286@redhat.com>
Date:   Tue, 16 May 2023 20:37:57 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Christie <michael.christie@...cle.com>,
        Christian Brauner <brauner@...nel.org>,
        Thorsten Leemhuis <linux@...mhuis.info>,
        nicolas.dichtel@...nd.com,
        Linux kernel regressions list <regressions@...ts.linux.dev>,
        hch@...radead.org, stefanha@...hat.com, jasowang@...hat.com,
        mst@...hat.com, sgarzare@...hat.com,
        virtualization@...ts.linux-foundation.org, konrad.wilk@...cle.com,
        linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH v11 8/8] vhost: use vhost_tasks for worker threads

On 05/16, Eric W. Biederman wrote:
>
> A kernel thread can block SIGKILL and that is supported.
>
> For a thread that is part of a process you can't block SIGKILL when the
> task is part of a user mode process.

Or SIGSTOP. Another thread can call do_signal_stop()->signal_wake_up/etc.

> There is this bit in complete_signal when SIGKILL is delivered to any
> thread in the process.
>
> 			t = p;
> 			do {
> 				task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
> 				sigaddset(&t->pending.signal, SIGKILL);
> 				signal_wake_up(t, 1);
> 			} while_each_thread(p, t);

That is why the latest version adds try_set_pending_sigkill(). No, no,
it is not that I think this is a good idea.

> For clarity that sigaddset(&t->pending.signal, SIGKILL);  Really isn't
> setting SIGKILL pending,

Hmm. it does? Nevermind.

> The important part of that code is that SIGNAL_GROUP_EXIT gets set.
> That indicates the entire process is being torn down.

Yes. and the same is true for io-thread even if it calls get_signal()
and dequeues SIGKILL and clears TIF_SIGPENDING.

> but in that case the vhost logic needs to act like a process, just
> like io_uring does.

confused... create_io_thread() creates a sub-thread too?

Although I never understood this logic. I can't even understand the usage
of lower_32_bits() in create_io_thread().

Oleg.