Re: [PATCH] Revert "exec: make de_thread() freezable (was: Re: Linux 4.20-rc4)

From: Pavel Machek <pavel@ucw.cz>
To: Michal Hocko <mhocko@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
	Chanho Min <chanho.min@lge.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH] Revert "exec: make de_thread() freezable (was: Re: Linux 4.20-rc4)
Date: Mon, 3 Dec 2018 15:45:36 +0100	[thread overview]
Message-ID: <20181203144536.GA15279@amd> (raw)
In-Reply-To: <20181203141737.GY31738@dhcp22.suse.cz>

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On Mon 2018-12-03 15:17:37, Michal Hocko wrote:
> On Mon 03-12-18 15:14:59, Pavel Machek wrote:
> > On Mon 2018-12-03 14:53:51, Michal Hocko wrote:
> > > On Mon 03-12-18 14:10:06, Pavel Machek wrote:
> > > > On Mon 2018-12-03 13:38:57, Michal Hocko wrote:
> > > > > On Mon 03-12-18 13:31:49, Oleg Nesterov wrote:
> > > > > > On 12/03, Michal Hocko wrote:
> > > > > > >
> > > > > > > Now, I wouldn't mind to revert this because the code is really old and
> > > > > > > we haven't seen many bug reports about failing suspend yet. But what is
> > > > > > > the actual plan to make this work properly?
> > > > > > 
> > > > > > I don't see a simple solution...
> > > > > > 
> > > > > > But we need to fix exec/de_thread anyway, then we can probably reconsider
> > > > > > this patch.
> > > > > 
> > > > > My concern is that de_thread fix might be too disruptive for stable
> > > > > kernels while we might want to have a simple enough fix for the the
> > > > > suspend issue in the meantime. That was actually the primary reason I've
> > > > > acked the hack even though I didn't like it.
> > > > 
> > > > Do we care about failing sleep in stable? Does someone hit the issue there?
> > > > 
> > > > This sounds like issue only Android is hitting, and they run very
> > > > heavily patched kernels, far away from mainline or stable.
> > > 
> > > But the underlying issue is the same and independent on their patches
> > > AFAIU. And is this really a common problem to care about in stable? I
> > > dunno to be honest but it sounds annoying for sure. Failing suspend is
> > > something that doesn't make your day when you are in hurry and want
> > > find out only later when your laptop heats up your bag ;)
> > 
> > In general, yes. In practice, if it happens 1 in 1000000 suspends, you
> > don't care that much (but Android cares).
> 
> This argument just doesn't make any sense. Rare bugs are maybe even
> more

I guess argumenting about this just does not make sense. Just bear in
mind -stable is not for theoretical problems.

> annoying because you do not expect them to happen. But I would be more
> interested to see whether they are any downside. Is there any actual
> risk to silence the lockup detector that you can see?

As someone else noticed:

a) the bug can be triggered outside suspend

b) the lockdep report is real. You'll still get suspend failure, but
you now need two processes to trigger it

> > Do we actually have reports of this happening for people outside
> > Android?
> 
> Not that I am aware of.

Good.
									Pavel

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