Re: [PATCH v3 3/3] sched, x86: Check that we're on the right stack in schedule and __might_sleep

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Andi Kleen <andi@firstfloor.org>, Borislav Petkov <bp@alien8.de>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Oleg Nesterov <oleg@redhat.com>, Tony Luck <tony.luck@intel.com>
Subject: Re: [PATCH v3 3/3] sched, x86: Check that we're on the right stack in schedule and __might_sleep
Date: Mon, 23 May 2016 18:48:20 -0700	[thread overview]
Message-ID: <CA+55aFysHFxi3+gM69rpj445QQ352ZRucjw23ZeTBj-azp6pqg@mail.gmail.com> (raw)
In-Reply-To: <CALCETrXw0-KCSrjBDeKSp8GZQ8hmJXqH+2=UVPurgL7cAK6iJw@mail.gmail.com>

On Mon, May 23, 2016 at 6:23 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>
>> Or we could just let ksoftirqd do its thing and stop raising
>> HARDIRQ_COUNT.  We could add a new preempt count field just for IST
>> (yuck).  We could try to hijack a different preempt count field
>> (NMI?).  But I kind of like the idea of just reinstating the original
>> patch of explicitly checking that we're on a safe stack in schedule
>> and __might_sleep, since that is the actual condition we care about.
>
> Ping?  I can still trigger this fairly easily on 4.6.

.. I haven't seen a patch from you, last I saw that was kind of what I expected.

That said, I still despise your patch. Why can't you just fix
"in_interrupt()" and be done with it. The original patch was like 50
lines of changes for somethinig that feels like it should be a
one-liner.

And no, we don't add idiotic new config symbols for things like "I
have this one-liner trivial arch helper". What we do is to just test
for such a helper with "#ifdef" (and if it's a inline function we do
#define xyz xyz" so that the #ifdef works).

So the original patch in this thread is still off the table,
especially since there was absolutely no explanation for why it should
be such a crazy complicated thing.

What exactly is it you are nervous about scheduling in NMI's? I agree
that that would be disastrous, but it's not supposed to actually
happen.

              Linus