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

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Date:	Thu, 27 Nov 2014 16:27:50 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'David Hildenbrand' <dahi@...ux.vnet.ibm.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	"Heiko Carstens" <heiko.carstens@...ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"schwidefsky@...ibm.com" <schwidefsky@...ibm.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"mingo@...nel.org" <mingo@...nel.org>
Subject: RE: [RFC 0/2] Reenable might_sleep() checks for might_fault() when
 atomic

From: David Hildenbrand [mailto:dahi@...ux.vnet.ibm.com]
> > From: David Hildenbrand
> > ...
> > > 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. I am not sure if a completely separated counter is even
> > > possible, increasing the size of thread_info.
> >
> > What about adding (say) 0x10000 for the more restrictive test?
> >
> > 	David
> >
> 
> You mean as part of the preempt counter?
> 
> The current layout (on my branch) is
> 
>  * PREEMPT_MASK:        0x000000ff
>  * SOFTIRQ_MASK:        0x0000ff00
>  * HARDIRQ_MASK:        0x000f0000
>  *     NMI_MASK:        0x00100000
>  * PREEMPT_ACTIVE:      0x00200000
> 
> I would have added
>  * PAGEFAULT_MASK:      0x03C00000

I'm not sure where you'd need to add the bits.

I think the above works because disabling 'HARDIRQ' implicitly
disables 'SOFTIRQ' and 'PREEMPT' (etc), so if 256+ threads
disable PREEMPT everything still works.

So if disabling pagefaults implies that pre-emption is disabled
(but SOFTIRQ is still allowed) then you need to insert your bit(s)
between 0xff00 and 0x00ff.
OTOH if disabling pre-emption implies that pagefaults are disabled
then you'd need to use the lsb and change all the above values.

Which makes me think that 'PREEMPT_ACTIVE' isn't right at all.
Two threads disabling NMIs (or 32  disabling HARDIRQ) won't DTRT.

OTOH I'm only guessing at how this is used.

	David



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