linux-kernel - Re: [RFC 0/2] Reenable might_sleep() checks for might

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Date:	Fri, 28 Nov 2014 08:34:54 +0100
From:	David Hildenbrand <dahi@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	linuxppc-dev@...ts.ozlabs.org, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, benh@...nel.crashing.org,
	paulus@...ba.org, akpm@...ux-foundation.org,
	schwidefsky@...ibm.com, mingo@...nel.org
Subject: Re: [RFC 0/2] Reenable might_sleep() checks for might_fault() when
 atomic

> On Thu, 27 Nov 2014, David Hildenbrand wrote:
> > > OTOH, there is no reason why we need to disable preemption over that
> > > page_fault_disabled() region. There are code pathes which really do
> > > not require to disable preemption for that.
> > > 
> > > We have that seperated in preempt-rt for obvious reasons and IIRC
> > > Peter Zijlstra tried to distangle it in mainline some time ago. I
> > > forgot why that never got merged.
> > > 
> > 
> > Of course, we can completely separate that in our page fault code by doing
> > pagefault_disabled() checks instead of in_atomic() checks (even in add on
> > patches later).
> > 
> > > We tie way too much stuff on the preemption count already, which is a
> > > mightmare because we have no clear distinction of protection
> > > scopes. 
> > 
> > Although it might not be optimal, but keeping a separate counter for
> > pagefault_disable() as part of the preemption counter seems to be the only
> > doable thing right now.
> 
> It needs to be seperate, if it should be useful. Otherwise we just
> have a extra accounting in preempt_count() which does exactly the same
> thing as we have now: disabling preemption.
> 
> Now you might say, that we could mask out that part when checking
> preempt_count, but that wont work on x86 as x86 has the preempt
> counter as a per cpu variable and not as a per thread one.

Ah right, it's per cpu on x86. So it really belongs to a thread if we want to
demangle preemption and pagefault_disable.

Would work for now, but for x86 not on the long run.

> 
> But if you want to distangle pagefault disable from preempt disable
> then you must move it to the thread, because it is a property of the
> thread. preempt count is very much a per cpu counter as you can only
> go through schedule when it becomes 0.

Thinking about it, this makes perfect sense!

> 
> Btw, I find the x86 representation way more clear, because it
> documents that preempt count is a per cpu BKL and not a magic thread
> property. And sadly that is how preempt count is used ...
> 
> > I am not sure if a completely separated counter is even possible,
> > increasing the size of thread_info.
> 
> And adding a ulong to thread_info is going to create exactly which
> problem?

If we're allowed to increase the size of thread_info - absolutely fine with me!
(I am not sure if some archs have special constraints on the size)

Will see what I can come up with.

Thanks!

> 
> Thanks,
> 
> 	tglx
> 

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