Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 16 May 2023 20:37:57 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Mike Christie <michael.christie@oracle.com>,
        Christian Brauner <brauner@kernel.org>,
        Thorsten Leemhuis <linux@leemhuis.info>,
        nicolas.dichtel@6wind.com,
        Linux kernel regressions list <regressions@lists.linux.dev>,
        hch@infradead.org, stefanha@redhat.com, jasowang@redhat.com,
        mst@redhat.com, sgarzare@redhat.com,
        virtualization@lists.linux-foundation.org, konrad.wilk@oracle.com,
        linux-kernel@vger.kernel.org, Jens Axboe <axboe@kernel.dk>
Subject: Re: [PATCH v11 8/8] vhost: use vhost_tasks for worker threads
Message-ID: <20230516183757.GA1286@redhat.com>
References: <aba6cca4-e66c-768f-375c-b38c8ba5e8a8@6wind.com>
 <CAHk-=wgadfsCnKHLON7op=Qs5t3w3PVz5ZDbvbKsfb=yBg=yjQ@mail.gmail.com>
 <78c5e150-26cf-7724-74ee-4a0b16b944b1@oracle.com>
 <48842e92-835e-bc3f-7118-48b8f415f532@leemhuis.info>
 <CAHk-=wicXCNR1bBioaAiBqEDgv-CoQD9z2RsM2VOTPzkK0xr8g@mail.gmail.com>
 <20230515-vollrausch-liebgeworden-2765f3ca3540@brauner>
 <CAHk-=wgXJ5VS1iBkfsG=HDjsyhn5XYDKt5xhQcNuz-e7VKyg8A@mail.gmail.com>
 <122b597e-a5fa-daf7-27bb-6f04fa98d496@oracle.com>
 <CAHk-=wgTpRDwS+F0Gd6+bM+TEh+x4Aiz8EBDTnTm3Q4TFdWOww@mail.gmail.com>
 <87cz30s20y.fsf@email.froward.int.ebiederm.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <87cz30s20y.fsf@email.froward.int.ebiederm.org>
User-Agent: Mutt/1.5.24 (2015-08-30)
Precedence: bulk

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.