Subject: Re: [PATCH v2 10/10] kernel: might_fault does not imply might_sleep

From: Peter Zijlstra <[email-masked]>
Date: Tue, 21 May 2013 13:57:34 +0200

On Sun, May 19, 2013 at 12:35:26PM +0300, Michael S. Tsirkin wrote:
> > > --- a/include/linux/kernel.h
> > > +++ b/include/linux/kernel.h
> > > @@ -198,7 +198,6 @@ void might_fault(void);
> > >  #else
> > >  static inline void might_fault(void)
> > >  {
> > > -	might_sleep();
> > 
> > This removes potential resched points for PREEMPT_VOLUNTARY -- was that
> > intentional?
> 
> No it's a bug. Thanks for pointing this out.
> OK so I guess it should be might_sleep_if(!in_atomic())
> and this means might_fault would have to move from linux/kernel.h to
> linux/uaccess.h, since in_atomic() is in linux/hardirq.h
> 
> Makes sense?

So the only difference between PROVE_LOCKING and not should be the
might_lock_read() thing; so how about something like this?

---
 include/linux/kernel.h  |  7 ++-----
 include/linux/uaccess.h | 26 ++++++++++++++++++++++++++
 mm/memory.c             | 14 ++------------
 3 files changed, 30 insertions(+), 17 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index e96329c..70812f4 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -194,12 +194,9 @@ extern int _cond_resched(void);
 	})
 
 #ifdef CONFIG_PROVE_LOCKING
-void might_fault(void);
+void might_fault_lockdep(void);
 #else
-static inline void might_fault(void)
-{
-	might_sleep();
-}
+static inline void might_fault_lockdep(void) { }
 #endif
 
 extern struct atomic_notifier_head panic_notifier_list;
diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 5ca0951..50a2cc9 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -38,6 +38,32 @@ static inline void pagefault_enable(void)
 	preempt_check_resched();
 }
 
+static inline bool __can_fault(void)
+{
+	/*
+	 * Some code (nfs/sunrpc) uses socket ops on kernel memory while
+	 * holding the mmap_sem, this is safe because kernel memory doesn't
+	 * get paged out, therefore we'll never actually fault, and the
+	 * below annotations will generate false positives.
+	 */
+	if (segment_eq(get_fs(), KERNEL_DS))
+		return false;
+
+	if (in_atomic() /* || pagefault_disabled() */)
+		return false;
+
+	return true;
+}
+
+static inline void might_fault(void)
+{
+	if (!__can_fault())
+		return;
+
+	might_sleep();
+	might_fault_lockdep();
+}
+
 #ifndef ARCH_HAS_NOCACHE_UACCESS
 
 static inline unsigned long __copy_from_user_inatomic_nocache(void *to,
diff --git a/mm/memory.c b/mm/memory.c
index 6dc1882..266610c 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4211,19 +4211,9 @@ void print_vma_addr(char *prefix, unsigned long ip)
 }
 
 #ifdef CONFIG_PROVE_LOCKING
-void might_fault(void)
+void might_fault_lockdep(void)
 {
 	/*
-	 * Some code (nfs/sunrpc) uses socket ops on kernel memory while
-	 * holding the mmap_sem, this is safe because kernel memory doesn't
-	 * get paged out, therefore we'll never actually fault, and the
-	 * below annotations will generate false positives.
-	 */
-	if (segment_eq(get_fs(), KERNEL_DS))
-		return;
-
-	might_sleep();
-	/*
 	 * it would be nicer only to annotate paths which are not under
 	 * pagefault_disable, however that requires a larger audit and
 	 * providing helpers like get_user_atomic.
@@ -4231,7 +4221,7 @@ void might_fault(void)
 	if (!in_atomic() && current->mm)
 		might_lock_read(&current->mm->mmap_sem);
 }
-EXPORT_SYMBOL(might_fault);
+EXPORT_SYMBOL(might_fault_lockdep);
 #endif
 
 #if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)