linux-kernel - Re: [PATCH 3/3] staging: r8188eu: don't accept SIGTERM for cmd thread

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Date:   Sun, 17 Oct 2021 20:02:37 +0200
From:   Martin Kaiser <lists@...ser.cx>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Phillip Potter <phil@...lpotter.co.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Larry Finger <Larry.Finger@...inger.net>,
        Michael Straube <straube.linux@...il.com>,
        linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] staging: r8188eu: don't accept SIGTERM for cmd thread

Hi Fabio and all,

Thus wrote Fabio M. De Francesco (fmdefrancesco@...il.com):

> On Sunday, October 17, 2021 12:29:02 PM CEST Phillip Potter wrote:

> > So I myself am a little confused on this one :-)

> > Based on my understanding, so correct me if I'm wrong, a process
> > (kthread or otherwise) can still be killed if marked TASK_KILLABLE,
> > even if ignoring SIGTERM. Indeed, from a userspace perspective,
> > SIGKILL is unblockable anyway - although of course kernel code can
> > choose how to respond to it.

> Correct.

And it seems that by default, a kthread can't be killed with SIGKILL.

> > So in other words, the kthread could still be killed while waiting
> > in the wait_for_completion_killable() call, even if we are ignoring
> > SIGTERM. From that perspective I guess, it is therefore not 'incorrect' as
> > such - if indeed we wanted that behaviour.

> No. This misunderstandings is my fault. :(

> In Martin's patch I read "SIGTERM" but for some reason I thought he was 
> talking of "SIGKILL".

> At the moment, without Martin's patch, the kthread can be terminated by the 
> command "kill -TERM <PID>". If we try "kill -KILL <PID>", nothing happens.
> This is because only "allow_signal(SIGTERM);" is present in the code.

Exactly. And this is probably not by intention. It would be consistent
to either allow both or none - the latter makes more sense, and it's
what most other drivers do.

> I think that kthreads must also allow  SIGKILL with "allow_signal(SIGKILL);" 
> for allowing root to make them terminate.

Probably. nfsd seems to do this.

> For what relates to my patch, it doesn't matter if I either leave 
> wait_for_completion_killable() as-is or change it to wait_for_completion().
> This is because at the moment SIGKILL cannot kill rtw_cmd_thread(), while 
> SIGTERM can.

> However, for consistency, I should better change it to the uninterruptible 
> version.

That makes sense to me.

Let's see what Greg and others say...

Best regards,
Martin