linux-kernel - Re: Linux 3.19-rc5

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Message-ID: <20150205221436.01bc24f2@neptune.home>
Date:	Thu, 5 Feb 2015 22:14:36 +0100
From:	Bruno Prémont <bonbons@...ux-vserver.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Hurley <peter@...leysoftware.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ilya Dryomov <ilya.dryomov@...tank.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: Linux 3.19-rc5

On Thu, 29 January 2015 Linus Torvalds wrote:
> On Thu, Jan 29, 2015 at 5:12 PM, Linus Torvalds wrote:
> >
> > The WARN() was already changed to a WARN_ONCE().
> 
> Oh, but I notice that the "__set_current_state(TASK_RUNNING) ends up
> always happening.
> 
> So I think the right fix is to:
> 
>  - warn once like we do
> 
>  - but *not* do that __set_current_state() which was always total crap anyway
> 
> Why do I say "total crap"? Because of two independent issues:
> 
>  (a) it actually changes behavior for a debug vs non-debug kernel,
> which is a really bad idea to begin with
> 
>  (b) it's really wrong. The whole "nested sleep" case was never a
> major bug to begin with, just a possible inefficiency where constant
> nested sleeps would possibly make the outer sleep not sleep. But that
> "could possibly make" case was the unlikely case, and the debug patch
> made it happen *all* the time by explicitly setting things running.
> 
> So I think the proper patch is the attached.
> 
> The comment is also crap. The comment says
> 
>     "Blocking primitives will set (and therefore destroy) current->state [...]"
> 
> but the reality is that they *may* set it, and only in the unlikely
> slow-path where they actually block.
> 
> So doing this in "__may_sleep()" is just bogus and horrible horrible
> crap. It turns the "harmless ugliness" into a real *harmful* bug. The
> key word of "__may_sleep()" is that "MAY" part. It's a debug thing to
> make relatively rare cases show up.
> 
> PeterZ, please don't make "debugging" patches like this. Ever again.
> Because this was just stupid, and it took me too long to realize that
> despite the warning being shut up, the debug patch was still actively
> doing bad bad things.
> 
> Ingo, maybe you'd want to apply this through the scheduler tree, the
> way you already did the WARN_ONCE() thing.
> 
> Bruno, does this finally actually fix your pccard thing?

Tested the variant that was applied by running rc7 and it fixes the
endless loop.
The loop is now replaced by a single WARN() trace - I guess expected:

[    3.083647] ------------[ cut here ]------------
[    3.087477] WARNING: CPU: 0 PID: 67 at /usr/src/linux-git/kernel/sched/core.c:7300 __might_sleep+0x79/0x90()
[    3.091357] do not call blocking ops when !TASK_RUNNING; state=1 set at [<c1442040>] pccardd+0xa0/0x3e0
[    3.095232] Modules linked in:
[    3.099020] CPU: 0 PID: 67 Comm: pccardd Not tainted 3.19.0-rc7-00003-g67288c4 #17
[    3.102760] Hardware name: Acer TravelMate 660/TravelMate 660, BIOS 3A19 01/14/2004
[    3.106504]  c212def4 c212def4 c212deb4 c16caf23 c212dee4 c10416fd c1907334 c212df10
[    3.110315]  00000043 c1907380 00001c84 c105a099 c105a099 c1442040 00000001 f5f54bc0
[    3.114143]  c212defc c104176e 00000009 c212def4 c1907334 c212df10 c212df30 c105a099
[    3.117960] Call Trace:
[    3.121703]  [<c16caf23>] dump_stack+0x16/0x18
[    3.125447]  [<c10416fd>] warn_slowpath_common+0x7d/0xc0
[    3.129172]  [<c105a099>] ? __might_sleep+0x79/0x90
[    3.132868]  [<c105a099>] ? __might_sleep+0x79/0x90
[    3.136500]  [<c1442040>] ? pccardd+0xa0/0x3e0
[    3.140092]  [<c104176e>] warn_slowpath_fmt+0x2e/0x30
[    3.143657]  [<c105a099>] __might_sleep+0x79/0x90
[    3.147209]  [<c1442040>] ? pccardd+0xa0/0x3e0
[    3.150747]  [<c1442040>] ? pccardd+0xa0/0x3e0
[    3.154256]  [<c16cf447>] mutex_lock+0x17/0x2a
[    3.157734]  [<c1442089>] pccardd+0xe9/0x3e0
[    3.161207]  [<c1441fa0>] ? pcmcia_socket_uevent+0x30/0x30
[    3.164660]  [<c1056990>] kthread+0xa0/0xc0
[    3.168059]  [<c16d1040>] ret_from_kernel_thread+0x20/0x30
[    3.171436]  [<c10568f0>] ? kthread_worker_fn+0x140/0x140
[    3.174796] ---[ end trace c3f708b642e3d8f0 ]---

>From my reading of the thread fixing pccardd/sched TASK_RUNNING usage/check
is another issue left for the future.

Thanks,
Bruno

>                               Linus
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