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

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Message-ID: <20141127090301.3ddc3077@thinkpad-w530>
Date:	Thu, 27 Nov 2014 09:03:01 +0100
From:	David Hildenbrand <dahi@...ux.vnet.ibm.com>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
Cc:	"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

> Code like
> 	spin_lock(&lock);
> 	if (copy_to_user(...))
> 		rc = ...
> 	spin_unlock(&lock);
> really *should* generate warnings like it did before.
> 
> And *only* code like
> 	spin_lock(&lock);

Is only code like this valid or also with the spin_lock() dropped?
(e.g. the access in patch1 if I remember correctly)


So should page_fault_disable() increment the pagefault counter and the preempt
counter or only the first one?

> 	page_fault_disable();
> 	if (copy_to_user(...))
> 		rc = ...
> 	page_fault_enable();
> 	spin_unlock(&lock);
> should not generate warnings, since the author hopefully knew what he did.
> 
> We could achieve that by e.g. adding a couple of pagefault disabled bits
> within current_thread_info()->preempt_count, which would allow
> pagefault_disable() and pagefault_enable() to modify a different part of
> preempt_count than it does now, so there is a way to tell if pagefaults have
> been explicitly disabled or are just a side effect of preemption being
> disabled.
> This would allow might_fault() to restore its old sane behaviour for the
> !page_fault_disabled() case.

So we would have pagefault code rely on:

in_disabled_pagefault() ( pagefault_disabled() ... whatever ) instead of
in_atomic().

I agree with this approach, as this is basically what I suggested in one of my
previous mails.

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