Re: [PATCH v11 8/8] vhost: use vhost_tasks for worker threads

From: Oleg Nesterov <oleg@redhat.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Jens Axboe <axboe@kernel.dk>,
	Christian Brauner <brauner@kernel.org>,
	Linux kernel regressions list <regressions@lists.linux.dev>,
	mst@redhat.com, linux-kernel@vger.kernel.org,
	konrad.wilk@oracle.com, Thorsten Leemhuis <linux@leemhuis.info>,
	virtualization@lists.linux-foundation.org, hch@infradead.org,
	stefanha@redhat.com, nicolas.dichtel@6wind.com,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH v11 8/8] vhost: use vhost_tasks for worker threads
Date: Wed, 17 May 2023 19:09:18 +0200	[thread overview]
Message-ID: <20230517170917.GA17016@redhat.com> (raw)
In-Reply-To: <87mt24ox2d.fsf@email.froward.int.ebiederm.org>

On 05/16, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@redhat.com> writes:
>
> >> 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.
>
> I see that try_set_pending_sigkill in the patch now.
>
> That try_set_pending_sigkill just keeps the process from reporting
> that it has exited, and extend the process exit indefinitely.
>
> SIGNAL_GROUP_EXIT has already been set, so the KILL signal was
> already delivered and the process is exiting.

Agreed, that is why I said I don't think try_set_pending_sigkill() is
a good idea.

And again, the same is true for the threads created by
create_io_thread(). get_signal() from io_uring/ can dequeue a pending
SIGKILL and return, but that is all.

> >> For clarity that sigaddset(&t->pending.signal, SIGKILL);  Really isn't
> >> setting SIGKILL pending,
> >
> > Hmm. it does? Nevermind.
>
> The point is that what try_set_pending_sigkill in the patch is doing is
> keeping the "you are dead exit now" flag, from being set.
>
> That flag is what fatal_signal_pending always tests, because we can only
> know if a fatal signal is pending if we have performed short circuit
> delivery on the signal.
>
> The result is the effects of the change are mostly what people expect.
> The difference the semantics being changed aren't what people think they
> are.
>
> AKA process exit is being ignored for the thread, not that SIGKILL is
> being blocked.

Sorry, I don't understand. I just tried to say that
sigaddset(&t->pending.signal, SIGKILL) really sets SIGKILL pending.
Nevermind.

> > Although I never understood this logic.

I meant I never really liked how io-threads play with signals,

> I can't even understand the usage
> > of lower_32_bits() in create_io_thread().
>
> As far as I can tell lower_32_bits(flags) is just defensive programming

Cough. but this is ugly. Or I missed something.

> or have just populated .flags directly.

Exactly,

> Then .exit_signal
> could have been set to 0.

Exactly.

-------------------------------------------------------------------------------
OK. It doesn't matter. I tried to read the whole thread and got lost.

IIUC, Mike is going to send the next version? So I think we can delay
the further discussions until then.

Oleg.

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