Date: Thu, 7 May 2015 13:25:01 +0200
From: Ingo Molnar <mingo@kernel.org>
To: David Hildenbrand <dahi@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org,
        mingo@redhat.com, yang.shi@windriver.com, bigeasy@linutronix.de,
        benh@kernel.crashing.org, paulus@samba.org, akpm@linux-foundation.org,
        heiko.carstens@de.ibm.com, schwidefsky@de.ibm.com,
        borntraeger@de.ibm.com, mst@redhat.com, tglx@linutronix.de,
        David.Laight@ACULAB.COM, hughd@google.com, hocko@suse.cz,
        ralf@linux-mips.org, herbert@gondor.apana.org.au,
        linux@arm.linux.org.uk, airlied@linux.ie, daniel.vetter@intel.com,
        linux-mm@kvack.org, linux-arch@vger.kernel.org
Subject: Re: [PATCH RFC 01/15] uaccess: count pagefault_disable() levels in
 pagefault_disabled
Message-ID: <20150507112501.GA15439@gmail.com>
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* David Hildenbrand <dahi@linux.vnet.ibm.com> wrote:

> > On Thu, May 07, 2015 at 12:50:53PM +0200, David Hildenbrand wrote:
> > > Just to make sure we have a common understanding (as written in my cover
> > > letter):
> > > 
> > > Your suggestion won't work with !CONFIG_PREEMPT (!CONFIG_PREEMPT_COUNT). If
> > > there is no preempt counter, in_atomic() won't work. 
> > 
> > But there is, we _always_ have a preempt_count, and irq_enter() et al.
> > _always_ increment the relevant bits.
> > 
> > The thread_info::preempt_count field it never under PREEMPT_COUNT
> > include/asm-generic/preempt.h provides stuff regardless of
> > PREEMPT_COUNT.
> > 
> > See how __irq_enter() -> preempt_count_add(HARDIRQ_OFFSET) ->
> > __preempt_count_add() _always_ just works.
> > 
> > Its only things like preempt_disable() / preempt_enable() that get
> > munged depending on PREEMPT_COUNT/PREEMPT.
> > 
> 
> Sorry for the confusion. Sure, there is always the count.
> 
> My point is that preempt_disable() won't result in an in_atomic() == true
> with !PREEMPT_COUNT, so I don't see any point in adding in to the pagefault
> handlers. It is not reliable.

That's why we have the preempt_count_inc()/dec() methods that are 
always available.

So where's the problem?

Thanks,

	Ingo
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