Re: [PATCH] signals: Generate warning when flush_signals() is called from non-kthread context

From: NeilBrown <neilb@suse.de>
To: Jiri Kosina <jkosina@suse.cz>
Cc: Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Evgeniy Polyakov <zbr@ioremap.net>,
	Stephen Smalley <sds@tycho.nsa.gov>,
	Alex Williamson <alex.williamson@redhat.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	kvm <kvm@vger.kernel.org>
Subject: Re: [PATCH] signals: Generate warning when flush_signals() is called from non-kthread context
Date: Fri, 8 May 2015 08:37:00 +1000	[thread overview]
Message-ID: <20150508083700.7a0ff4f8@notabene.brown> (raw)
In-Reply-To: <alpine.LRH.2.00.1505071531430.11268@twin.jikos.cz>

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On Thu, 7 May 2015 15:33:53 +0200 (CEST) Jiri Kosina <jkosina@suse.cz> wrote:

> On Mon, 4 May 2015, Oleg Nesterov wrote:
> 
> > > All the calls in md.c are in a kernel thread so safe, but I'd rather have an
> > > explicit "uninterruptible, but no load-average" wait....
> > 
> > Could you please explain why md_thread() does allow_signal(SIGKILL) ?
> > 
> > I am just curious. It looks as if we want to allow user-space to "call"
> > thread->run(), and this looks strange.
> 
> One would think that this is because md wants to be notified when system 
> is going to be halted/rebooted, and userspace init (whatever that is) 
> decides to do 'kill -9 -1' to perform the final shutdown of md (the 
> question is why it really should be needed, becasue all filesystems should 
> be R/O by that time anyway).
> 

Something like that.  It is historical strangeness that might have seemed
like a good idea at the time, but is hard to justify.

There is an alt-sysrq that will remount filesystems read-only, but no
alt-sysrq to switch RAID arrays to "idle/clean".  So I leverages alt-sysrq-K,
which does "kill -9 -1" (I think).

Since md gained the ability to switch to idle/clean after a short timeout,
and also the ability to use a write-intent-bitmap so only little bits of the
array are ever non idle/clean, this all became much less interesting.

NeilBrown

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