[tip:sched/mm] mm, sched: Allow uaccess in atomic with pagefault_disable()

From: tip-bot for Michael S. Tsirkin
Date: Tue May 28 2013 - 09:27:55 EST

Commit-ID: 662bbcb2747c2422cf98d3d97619509379eee466
Gitweb: http://git.kernel.org/tip/662bbcb2747c2422cf98d3d97619509379eee466
Author: Michael S. Tsirkin <mst@xxxxxxxxxx>
AuthorDate: Sun, 26 May 2013 17:32:23 +0300
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 28 May 2013 09:41:11 +0200

mm, sched: Allow uaccess in atomic with pagefault_disable()

This changes might_fault() so that it does not
trigger a false positive diagnostic for e.g. the following
sequence:

spin_lock_irqsave()
pagefault_disable()
copy_to_user()
pagefault_enable()
spin_unlock_irqrestore()

In particular vhost wants to do this, to call
socket ops from under a lock.

There are 3 cases to consider:

- CONFIG_PROVE_LOCKING - might_fault is non-inline
so it's easy to move the in_atomic test to fix
up the false positive warning.

- CONFIG_DEBUG_ATOMIC_SLEEP - might_fault
is currently inline, but we are calling a
non-inline __might_sleep anyway,
so let's use the non-line version of might_fault
that does the right thing.

- !CONFIG_DEBUG_ATOMIC_SLEEP && !CONFIG_PROVE_LOCKING
__might_sleep is a nop so might_fault is a nop.

Make this explicit.

Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Link: http://lkml.kernel.org/r/1369577426-26721-11-git-send-email-mst@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
include/linux/kernel.h | 7 ++-----
mm/memory.c | 11 +++++++----
2 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 24719ea..4c7e2e5 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -193,13 +193,10 @@ extern int _cond_resched(void);
(__x < 0) ? -__x : __x; \
})

-#ifdef CONFIG_PROVE_LOCKING
+#if defined(CONFIG_PROVE_LOCKING) || defined(CONFIG_DEBUG_ATOMIC_SLEEP)
void might_fault(void);
#else
-static inline void might_fault(void)
-{
- __might_sleep(__FILE__, __LINE__, 0);
-}
+static inline void might_fault(void) { }
#endif

extern struct atomic_notifier_head panic_notifier_list;
diff --git a/mm/memory.c b/mm/memory.c
index c1f190f..d7d54a1 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4210,7 +4210,7 @@ void print_vma_addr(char *prefix, unsigned long ip)
up_read(&mm->mmap_sem);
}

-#ifdef CONFIG_PROVE_LOCKING
+#if defined(CONFIG_PROVE_LOCKING) || defined(CONFIG_DEBUG_ATOMIC_SLEEP)
void might_fault(void)
{
/*
@@ -4222,14 +4222,17 @@ void might_fault(void)
if (segment_eq(get_fs(), KERNEL_DS))
return;

- __might_sleep(__FILE__, __LINE__, 0);
-
/*
* 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.
*/
- if (!in_atomic() && current->mm)
+ if (in_atomic())
+ return;
+
+ __might_sleep(__FILE__, __LINE__, 0);
+
+ if (current->mm)
might_lock_read(&current->mm->mmap_sem);
}
EXPORT_SYMBOL(might_fault);
--
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