Commit 0c871971 authored Jun 15, 2009 by Ingo Molnar

perf_counter, x86: Improve interactions with fast-gup



Improve a few details in perfcounter call-chain recording that
makes use of fast-GUP:

- Use ACCESS_ONCE() to observe the pte value. ptes are fundamentally
  racy and can be changed on another CPU, so we have to be careful
  about how we access them. The PAE branch is already careful with
  read-barriers - but the non-PAE and 64-bit side needs an
  ACCESS_ONCE() to make sure the pte value is observed only once.

- make the checks a bit stricter so that we can feed it any kind of
  cra^H^H^H user-space input ;-)

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

parent e5289d4a

arch/x86/include/asm/uaccess.h

+6 −1

Original line number	Diff line number	Diff line
		@@ -25,7 +25,12 @@
		#define MAKE_MM_SEG(s) ((mm_segment_t) { (s) })

		#define KERNEL_DS MAKE_MM_SEG(-1UL)

		#ifdef CONFIG_X86_32
		# define USER_DS MAKE_MM_SEG(PAGE_OFFSET)
		#else
		# define USER_DS MAKE_MM_SEG(__VIRTUAL_MASK)
		#endif

		#define get_ds() (KERNEL_DS)
		#define get_fs() (current_thread_info()->addr_limit)

arch/x86/mm/gup.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -14,7 +14,7 @@
		static inline pte_t gup_get_pte(pte_t *ptep)
		{
		#ifndef CONFIG_X86_PAE
		return *ptep;
		return ACCESS_ONCE(*ptep);
		#else
		/*
		* With get_user_pages_fast, we walk down the pagetables without taking

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