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SubjectRe: [PATCH] mm: pagefault_{disable,enable}()
On Thu, 7 Dec 2006, Linux Kernel Mailing List wrote:
> Gitweb: http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=a866374aecc90c7d90619727ccd851ac096b2fc7
> Commit: a866374aecc90c7d90619727ccd851ac096b2fc7
> Parent: 6edaf68a87d17570790fd55f0c451a29ec1d6703
> Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
> AuthorDate: Wed Dec 6 20:32:20 2006 -0800
> Committer: Linus Torvalds <torvalds@woody.osdl.org>
> CommitDate: Thu Dec 7 08:39:21 2006 -0800
>
> [PATCH] mm: pagefault_{disable,enable}()
>
> Introduce pagefault_{disable,enable}() and use these where previously we did
> manual preempt increments/decrements to make the pagefault handler do the
> atomic thing.
>
> Currently they still rely on the increased preempt count, but do not rely on
> the disabled preemption, this might go away in the future.
>
> (NOTE: the extra barrier() in pagefault_disable might fix some holes on
> machines which have too many registers for their own good)
>
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index a48d7f1..67918c2 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -1,8 +1,43 @@
> #ifndef __LINUX_UACCESS_H__
> #define __LINUX_UACCESS_H__
>
> +#include <linux/preempt.h>
> #include <asm/uaccess.h>
>
> +/*
> + * These routines enable/disable the pagefault handler in that
> + * it will not take any locks and go straight to the fixup table.
> + *
> + * They have great resemblance to the preempt_disable/enable calls
> + * and in fact they are identical; this is because currently there is
> + * no other way to make the pagefault handlers do this. So we do
> + * disable preemption but we don't necessarily care about that.
> + */
> +static inline void pagefault_disable(void)
> +{
> + inc_preempt_count();
> + /*
> + * make sure to have issued the store before a pagefault
> + * can hit.
> + */
> + barrier();
> +}
> +
> +static inline void pagefault_enable(void)
> +{
> + /*
> + * make sure to issue those last loads/stores before enabling
> + * the pagefault handler again.
> + */
> + barrier();
> + dec_preempt_count();
> + /*
> + * make sure we do..
> + */
> + barrier();
> + preempt_check_resched();
> +}
> +
> #ifndef ARCH_HAS_NOCACHE_UACCESS
>
> static inline unsigned long __copy_from_user_inatomic_nocache(void *to,

This change causes lots of compile errors of the following form on m68k:

| linux-2.6.20-rc4/include/linux/uaccess.h: In function `pagefault_disable':
| linux-2.6.20-rc4/include/linux/uaccess.h:18: error: dereferencing pointer to incomplete type
| linux-2.6.20-rc4/include/linux/uaccess.h: In function `pagefault_enable':
| linux-2.6.20-rc4/include/linux/uaccess.h:33: error: dereferencing pointer to incomplete type

-> inc_preempt_count
-> add_preempt_count
-> preempt_count
-> current_thread_info
-> task_thread_info
which needs the definition of struct task_struct.

The patch below fixes it by including <linux/sched.h> in
include/linux/uaccess.h. But perhaps this is the right time to move
struct task_struct to its own include instead?

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 67918c2..ae73f32 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -2,6 +2,7 @@
#define __LINUX_UACCESS_H__

#include <linux/preempt.h>
+#include <linux/sched.h>
#include <asm/uaccess.h>

/*
Gr{oetje,eeting}s,
Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
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